sousa-gecko/intl/l10n/docs/fluent/tutorial.md

```{eval-rst}
.. role:: html(code)
   :language: html
```

```{eval-rst}
.. role:: js(code)
   :language: JavaScript
```

# Fluent for Firefox Developers

This tutorial is intended for Firefox engineers already familiar with the previous
localization systems offered by Gecko - DTD and StringBundle - and assumes
prior experience with those systems.

For a more hands-on tutorial of understanding Fluent from the ground up, try
following the [Fluent DOMLocalization Tutorial](https://projectfluent.org/dom-l10n-documentation/), which provides some background on
how Fluent works and walks you through creating a basic web project from scratch that
uses Fluent for localization.

## Using Fluent in Gecko

[Fluent] is a modern localization system introduced into
the Gecko platform with a focus on quality, performance, maintenance and completeness.

The legacy DTD system is deprecated, and Fluent should be used where possible.

### Getting a Review

If you work on any patch that touches FTL files, you'll need to get a review
from [fluent-reviewers](https://phabricator.services.mozilla.com/tag/fluent-reviewers/). There's a Herald hook that automatically sets
that group as a blocking reviewer.

Guidelines for the review process are available [here](./review.md).

To lighten the burden on reviewers, please take a moment to review some
best practices before submitting your patch for review.

- [ProjectFluent Good Practices for Developers]
- [Mozilla Localization Best Practices For Developers]

## Major Benefits

Fluent [ties tightly](https://github.com/projectfluent/fluent/wiki/Fluent-and-Standards) into the domain of internationalization
through [Unicode], [CLDR] and [ICU].

More specifically, the most observable benefits for each group of consumers are

### Developers

- Support for XUL, XHTML, HTML, Web Components, React, JS, Python and Rust
- Strings are available in a single, unified localization context available for both DOM and runtime code
- Full internationalization (i18n) support: date and time formatting, number formatting, plurals, genders etc.
- Strong focus on [declarative API via DOM attributes](https://github.com/projectfluent/fluent/wiki/Get-Started)
- Extensible with custom formatters, Mozilla-specific APIs etc.
- [Separation of concerns](https://github.com/projectfluent/fluent/wiki/Design-Principles): localization details, and the added complexity of some languages, don't leak onto the source code and are no concern for developers
- Compound messages link a single translation unit to a single UI element
- [DOM Overlays](https://github.com/projectfluent/fluent.js/wiki/DOM-Overlays) allow for localization of DOM fragments
- Simplified build system model
- No need for pre-processing instructions
- Support for pseudolocalization

### Product Quality

- A robust, multilevel, [error fallback system](https://github.com/projectfluent/fluent/wiki/Error-Handling) prevents XML errors and runtime errors
- Simplified l10n API reduces the amount of l10n specific code and resulting bugs
- Runtime localization allows for dynamic language changes and updates over-the-air
- DOM Overlays increase localization security

## Fluent Translation List - FTL

Fluent introduces a file format designed specifically for easy readability
and the localization features offered by the system.

At first glance the format is a simple key-value store. It may look like this:

```fluent
home-page-header = Home Page

# The label of a button opening a new tab
new-tab-open = Open New Tab
```

But the FTL file format is significantly more powerful and the additional features
quickly add up. In order to familiarize yourself with the basic features,
consider reading through the [Fluent Syntax Guide] to understand
a more complex example like:

```fluent
### These messages correspond to security and privacy user interface.
###
### Please choose simple and non-threatening language when localizing
### to help user feel in control when interacting with the UI.

## General Section

-brand-short-name = Firefox
    .gender = masculine

pref-pane =
    .title =
        { PLATFORM() ->
            [windows] Options
           *[other] Preferences
        }
    .accesskey = C

# Variables:
#   $tabCount (Number) - number of container tabs to be closed
containers-disable-alert-ok-button =
    { $tabCount ->
        [one] Close { $tabCount } Container Tab
       *[other] Close { $tabCount } Container Tabs
    }

update-application-info =
    You are using { -brand-short-name } Version: { $version }.
    Please read the <a>privacy policy</a>.
```

The above, of course, is a particular selection of complex strings intended to exemplify
the new features and concepts introduced by Fluent.

:::{important}
While in Fluent it’s possible to use both lowercase and uppercase characters in message
identifiers, the naming convention in Gecko is to use lowercase and hyphens, avoiding
CamelCase and underscores. For example, `allow-button` should be preferred to
`allow_button` or `allowButton`, unless there are technically constraints – like
identifiers generated at run-time from external sources – that make this impractical.
:::

In order to ensure the quality of the output, a lot of checks and tooling
is part of the build system.
[Pontoon], the main localization tool used to translate Firefox, also supports
Fluent and its features to help localizers in their work.

(fluent-tutorial-social-contract)=

## Social Contract

Fluent uses the concept of a `social contract` between developer and localizers.
This contract is established by the selection of a unique identifier, called {js}`l10n-id`,
which carries a promise of being used in a particular place to carry a particular meaning.

The use of unique identifiers is shared with legacy localization systems in
Firefox.

:::{important}
An important part of the contract is that the developer commits to treat the
localization output as `opaque`. That means that no concatenations, replacements
or splitting should happen after the translation is completed to generate the
desired output.
:::

In return, localizers enter the social contract by promising to provide an accurate
and clean translation of the messages that match the request.

In Fluent, the developer is not to be bothered with inner logic and complexity that the
localization will use to construct the response. Whether [declensions](https://en.wikipedia.org/wiki/Declension) or other
variant selection techniques are used is up to a localizer and their particular translation.
From the developer perspective, Fluent returns a final string to be presented to
the user, with no l10n logic required in the running code.

## Markup Localization

To localize an element in Fluent, the developer adds a new message to
an FTL file and then has to associate an {js}`l10n-id` with the element
by defining a {js}`data-l10n-id` attribute:

```html
<h1 data-l10n-id="home-page-header" />

<button data-l10n-id="pref-pane" />
```

Fluent will take care of the rest, populating the element with the message value
in its content and all localizable attributes if defined.

The developer provides only a single message to localize the whole element,
including the value and selected attributes.

The value can be a whole fragment of DOM:

```html
<p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'>
  <a data-l10n-name="privacy-url" href="http://www.mozilla.org/privacy" />
</p>
```

```fluent
-brand-short-name = Firefox
update-application-info =
    You are using { -brand-short-name } Version: { $version }.
    Please read the <a data-l10n-name="privacy-url">privacy policy</a>.
```

Fluent will overlay the translation onto the source fragment preserving attributes like
{code}`class` and {code}`href` from the source and adding translations for the elements
inside. The resulting localized content will look like this:

```html
<p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'>
  You are using Firefox Version: 60.0.
  Please read the <a href="http://www.mozilla.org/privacy">privacy policy</a>.
</p>
```

This operation is sanitized, and Fluent takes care of selecting which elements and
attributes can be safely provided by the localization.
The list of allowed elements and attributes is [maintained by the W3C](https://www.w3.org/TR/2011/WD-html5-20110525/text-level-semantics.html), and if
the developer needs to allow for localization of additional attributes, they can
allow them using {code}`data-l10n-attrs` list:

```html
<label data-l10n-id="search-input" data-l10n-attrs="style" />
```

The above example adds an attribute {code}`style` to be allowed on this
particular {code}`label` element.

### External Arguments

Notice in the previous example the attribute {code}`data-l10n-args`, which is
a JSON object storing variables exposed by the developer to the localizer.

This is the main channel for the developer to provide additional variables
to be used in the localization.

It's worth noting that, when the {code}`l10n-args` are set in
the runtime code, they are in fact encoded as JSON and stored together with
{code}`l10n-id` as an attribute of the element.

## Runtime Localization

In almost every case the JS runtime code will operate on a particular document, either
XUL, XHTML or HTML.

If the document has its markup already localized, then Fluent exposes a new
attribute on the {js}`document` element - {js}`document.l10n`.

This property is an object of type {js}`DOMLocalization` which maintains the main
localization context for this document and exposes it to runtime code as well.

With a focus on [declarative localization](https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers), the primary method of localization is
to alter the localization attributes in the DOM. Fluent provides a method to facilitate this:

```JavaScript
document.l10n.setAttributes(element, "new-panel-header");
```

This will set the {code}`data-l10n-id` on the element and translate it before the next
animation frame.

This API can be used to set both the ID and the arguments at the same time.

```JavaScript
document.l10n.setAttributes(element, "containers-disable-alert-ok-button", {
  tabCount: 5
});
```

If only the arguments need to be updated, then it's possible to use the {code}`setArgs`
method.

```JavaScript
document.l10n.setArgs(element, {
  tabCount: 5
});
```

On debug builds if the Fluent arguments are not provided, then Firefox will crash. This
is done so that these errors are caught in CI. On rare occasions it may be necessary
to work around this crash by providing a blank string as an argument value.

### Non-Markup Localization

In rare cases, when the runtime code needs to retrieve the translation and not
apply it onto the DOM, Fluent provides an API to retrieve it:

```JavaScript
let [ msg ] = await document.l10n.formatValues([
  {id: "remove-containers-description"}
]);

alert(msg);
```

This model is heavily discouraged and should be used only in cases where the
DOM annotation is not possible.

:::{note}
This API is available as asynchronous. In case of Firefox,
the only non-DOM localizable calls are used where the output goes to
a third-party like Bluetooth, Notifications etc.
All those cases should already be asynchronous. If you can't avoid synchronous
access, you can use `mozILocalization.formatMessagesSync` with synchronous IO.
:::

## Internationalization

The majority of internationalization issues are implicitly handled by Fluent without
any additional requirement. Full Unicode support, [bidirectionality](https://github.com/projectfluent/fluent/wiki/BiDi-in-Fluent), and
correct number formatting work without any action required from either
developer or localizer.

```JavaScript
document.l10n.setAttributes(element, "welcome-message", {
  userName: "اليسع",
  count: 5
});
```

A message like this localized to American English will correctly wrap the user
name in directionality marks, allowing the layout engine to determine how to
display the bidirectional text.

On the other hand, the same message localized to Arabic will use the Eastern Arabic
numeral for number "5".

### Plural Rules

The most common localization feature is the ability to provide different variants
of the same string depending on plural categories. Fluent ties into the Unicode CLDR
standard called [Plural Rules](http://cldr.unicode.org/index/cldr-spec/plural-rules).

In order to allow localizers to use it, all the developer has to do is to pass
an external argument number:

```JavaScript
document.l10n.setAttributes(element, "unread-warning", { unreadCount: 5 });
```

Localizers can use the argument to build a multi variant message if their
language requires that:

```fluent
unread-warning =
    { $unreadCount ->
        [one] You have { $unreadCount } unread message
       *[other] You have { $unreadCount } unread messages
    }
```

If the variant selection is performed based on a number, Fluent matches that
number against literal numbers as well as its [plural category](https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html).

If the given translation doesn't need pluralization for the string (for example
Japanese often will not), the localizer can replace it with:

```fluent
unread-warning = You have { $unreadCount } unread messages
```

and the message will preserve the social contract.

One additional feature is that the localizer can further improve the message by
specifying variants for particular values:

```fluent
unread-warning =
    { $unreadCount ->
        [0] You have no unread messages
        [1] You have one unread message
       *[other] You have { $unreadCount } unread messages
    }
```

The advantage here is that per-locale choices don't leak onto the source code
and the developer is not affected.

:::{note}
There is an important distinction between a variant keyed on plural category
`one` and digit `1`. Although in English the two are synonymous, in other
languages category `one` may be used for other numbers.
For example in [Bosnian](https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html#bs), category `one` is used for numbers like `1`, `21`, `31`
and so on, and also for fractional numbers like `0.1`.
:::

### Partially-formatted variables

When it comes to formatting data, Fluent allows the developer to provide
a set of parameters for the formatter, and the localizer can fine tune some of them.
This technique is called [partially-formatted variables](https://projectfluent.org/fluent/guide/functions.html#partially-formatted-variables).

For example, when formatting a date, the developer can just pass a JS {js}`Date` object,
but its default formatting will be pretty expressive. In most cases, the developer
may want to use some of the {js}`Intl.DateTimeFormat` options to select the default
representation of the date in string:

```JavaScript
document.l10n.formatValue("welcome-message", {
startDate: FluentDateTime(new Date(), {
    year: "numeric",
    month: "long",
    day: "numeric"
  })
});
```

```fluent
welcome-message = Your session will start date: { $startDate }
```

In most cases, that will be enough and the date would get formatted in the current
Firefox as `February 28, 2018`.

But if in some other locale the string would get too long, the localizer can fine
tune the options as well:

```fluent
welcome-message = Początek Twojej sesji: { DATETIME($startDate, month: "short") }
```

This will adjust the length of the month token in the message to short and get formatted
in Polish as `28 lut 2018`.

At the moment Fluent supports two formatters that match JS Intl API counterparts:

- **NUMBER**: [Intl.NumberFormat](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NumberFormat)
- **DATETIME**: [Intl.DateTimeFormat](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat)

With time more formatters will be added. Also, this feature is not exposed
to `setAttributes` at this point, as that serializes to JSON.

## Registering New L10n Files

Fluent uses a wildcard statement, packaging all localization resources into
their component's `/localization/` directory.

That means that, if a new file is added to a component of Firefox already
covered by Fluent like `browser`, it's enough to add the new file to the
repository in a path like `browser/locales/en-US/browser/component/file.ftl`, and
the toolchain will package it into `browser/localization/browser/component/file.ftl`.

At runtime Firefox uses a special registry for all localization data. It will
register the browser's `/localization/` directory and make all files inside it
available to be referenced.

To make the document localized using Fluent, all the developer has to do is add
localizable resources for Fluent API to use:

```html
<link rel="localization" href="branding/brand.ftl"/>
<link rel="localization" href="browser/preferences/preferences.ftl"/>
```

The URI provided to the {html}`<link/>` element are relative paths within the localization
system.

## Custom Localizations

The above method creates a single localization context per document.
In almost all scenarios that's sufficient.

In rare edge cases where the developer needs to fetch additional resources, or
the same resources in another language, it is possible to create additional
Localization object manually using the `Localization` class:

```JavaScript
const myL10n = new Localization([
  "branding/brand.ftl",
  "browser/preferences/preferences.ftl"
]);


let [isDefaultMsg, isNotDefaultMsg] =
  await myL10n.formatValues({id: "is-default"}, {id: "is-not-default"});
```

:::{admonition} Example
An example of a use case is the Preferences UI in Firefox, which uses the
main context to localize the UI but also to build a search index.

It is common to build such search index both in a current language and additionally
in English, since a lot of documentation and online help exist only in English.

A developer may create manually a new context with the same resources as the main one,
but hardcode it to `en-US` and then build the search index using both contexts.
:::

By default, all `Localization` contexts are asynchronous. It is possible to create a synchronous
one by passing an `sync = false` argument to the constructor, or calling the `SetIsSync(bool)` method
on the class.

```JavaScript
const myL10n = new Localization([
  "branding/brand.ftl",
  "browser/preferences/preferences.ftl"
], false);


let [isDefaultMsg, isNotDefaultMsg] =
  myL10n.formatValuesSync({id: "is-default"}, {id: "is-not-default"});
```

Synchronous contexts should be always avoided as they require synchronous I/O. If you think your use case
requires a synchronous localization context, please consult Gecko, Performance and Localization teams.

## Designing Localizable APIs

When designing localizable APIs, the most important rule is to resolve localization as
late as possible. That means that instead of resolving strings somewhere deep in the
codebase and then passing them on, or even caching, it is highly recommended to pass
around {code}`l10n-id` or {code}`[l10n-id, l10n-args]` pairs until the top-most code
resolves them or applies them onto the DOM element.

## Testing

When writing tests that involve both I18n and L10n, the general rule is that
result strings are opaque. That means that the developer should not assume any particular
value and should never test against it.

In case of raw i18n the {js}`resolvedOptions` method on all {js}`Intl.*` formatters
makes it relatively easy. In case of localization, the recommended way is to test that
the code sets the right {code}`l10n-id`/{code}`l10n-args` attributes like this:

```JavaScript
testedFunction();

const l10nAttrs = document.l10n.getAttributes(element);

deepEquals(l10nAttrs, {
  id: "my-expected-id",
  args: {
    unreadCount: 5
  }
});
```

If the code really has to test for particular values in the localized UI, it is
always better to scan for a variable:

```JavaScript
testedFunction();

equals(element.textContent.contains("John"));
```

:::{important}
Testing against whole values is brittle and will break when we insert Unicode
bidirectionality marks into the result string or adapt the output in other ways.
:::

## Manually Testing UI with Pseudolocalization

When working with a Fluent-backed UI, the developer gets a new tool to test their UI
against several classes of problems.

Pseudolocalization is a mechanism which transforms messages on the fly, using
specific logic to help emulate how the UI will look once it gets localized.

The three classes of potential problems that this can help with are:

- Hardcoded strings.

  Turning on pseudolocalization should expose any strings that were left
  hardcoded in the source, since they won't get transformed.

- UI space not adapting to longer text.

  Many languages use longer strings than English. For example, German strings
  may be 30% longer (or more). Turning on pseudolocalization is a quick way to
  test how the layout handles such locales. Strings that don't fit the space
  available are truncated and pseudolocalization can also help with detecting them.

- Bidi adaptation.

  For many developers, testing the UI in right-to-left mode is hard.
  Pseudolocalization shows how a right-to-left locale will look like.

To turn on pseudolocalization, open the {doc}`Browser Toolbox <../../devtools-user/browser_toolbox/index>`,
click the three dot menu in the top right corner, and choose one of the following:

- **Enable “accented” locale** - [Ȧȧƈƈḗḗƞŧḗḗḓ Ḗḗƞɠŀīīşħ]

  This strategy replaces all Latin characters with their accented equivalents,
  and duplicates some vowels to create roughly 30% longer strings. Strings are
  wrapped in markers (square brackets), which help with detecting truncation.

  This option sets the `intl.l10n.pseudo` pref to `accented`.

- **Enable bidi locale** - ɥsıʅƃuƎ ıpıԐ

  This strategy replaces all Latin characters with their 180 degree rotated versions
  and enforces right to left text flow using Unicode UAX#9 [Explicit Directional Embeddings](https://www.unicode.org/reports/tr9/#Explicit_Directional_Embeddings).
  In this mode, the UI directionality will also be set to right-to-left.

  This option sets the `intl.l10n.pseudo` pref to `bidi`.

## Testing other locales

:::{important}
For Firefox engineering work, you should prefer using pseudolocales.
Especially on Nightly, localizations can be incomplete (as we add/remove
localized content all the time) and cause confusing behaviour due to how
fallback works.
:::

### Installing Nightly in a different locale

Localized Nightly builds are listed on [firefox.com].

### Installing language packs on local builds

To fix bugs that only reproduce with a specific locale, you may need to run a
development or nightly build with that locale. The UI language switcher in
Settings is disabled by default on Nightly, because language packs can become
incomplete and cause errors in the UI — there is no fallback to English for
strings using legacy formats, like .properties.

However, if you really need to use this, you can:

1. Open `about:config` and flip the `intl.multilingual.enabled` and
   `intl.multilingual.liveReload` preferences to `true`

2. Open [the FTP listing for langpacks] and click the XPI file corresponding
   to your language and nightly version (note that, especially around merge days,
   multiple versions may be present).

   :::{note}
   This is a Linux listing because that's the platform on which we run the
   l10n jobs, but the XPIs should work on macOS and Windows as well.
   The only exception is the "special" Japanese-for-mac locale,
   which is in the `mac/xpi` subdirectory under
   `latest-mozilla-central-l10n` instead. (`ja-JP-mac` and `ja` will
   both "work" cross-platform, but use different terminology in some places.)
   :::

3. Click through the prompts to install the language pack.

4. Open the Firefox Settings UI.

5. Switch to your chosen language.

### Building localized builds for single and multi-locale repacks

Instructions on how to build localization repacks are available on {doc}`/build/buildsystem/locales`.

### Including specific localization repacks to a Try Server push

Instructions are available in the [Firefox L10N FAQs].

### Finding a regression in a localized build

You can run [mozregression] with localized builds!

At the commandline, if you wanted to find a regression in a Dutch (`nl`)
build, you could run something like:

```
mozregression --app firefox-l10n --lang nl --good 2024-01-01
```

and that should run localized nightlies.

## Inner Structure of Fluent

The inner structure of Fluent in Gecko is out of scope of this tutorial, but
since the class and file names may show up during debugging or profiling,
below is a list of major components, each with a corresponding file in `/intl/l10n`
modules in Gecko.

For more hands-on experience with some of the concepts below, try
following the [Fluent DOMLocalization Tutorial](https://projectfluent.org/dom-l10n-documentation/overview.html), which provides some
background on how Fluent works and walks you through creating a basic
web project from scratch that uses Fluent for localization.

### FluentBundle

FluentBundle is the lowest level API. It's fully synchronous, contains a parser for the
FTL file format and a resolver for the logic. It is not meant to be used by
consumers directly.

In the future we intend to offer this layer for standardization and it may become
part of the {js}`mozIntl.*` or even {js}`Intl.*` API sets.

That part of the codebase is also the first that we'll be looking to port to Rust.

### Localization

Localization is a higher level API which uses {js}`FluentBundle` internally but
provides a full layer of compound message formatting and robust error fall-backing.

It is intended for use in runtime code and contains all fundamental localization
methods.

### DOMLocalization

DOMLocalization extends {js}`Localization` with functionality to operate on HTML, XUL
and the DOM directly including DOM Overlays and Mutation Observers.

### DocumentL10n

DocumentL10n implements the DocumentL10n WebIDL API and allows Document to
communicate with DOMLocalization.

#### Events

DOM translation is asynchronous (e.g., setting a `data-l10n-id` attribute won't
immediately reflect the localized content in the DOM).

We expose a {js}`Document.hasPendingL10nMutations` member that reflects whether
there are any async operations pending. When they are finished, the
`L10nMutationsFinished` event is fired on the document, so that chrome code can
be certain all the async operations are done.

### L10nRegistry

L10nRegistry is our resource management service. It
maintains the state of resources packaged into the build and language packs,
providing an asynchronous iterator of {js}`FluentBundle` objects for a given locale set
and resources that the {js}`Localization` class uses.

[cldr]: http://cldr.unicode.org/
[firefox l10n faqs]: https://mozilla-l10n.github.io/documentation/products/firefox_desktop/firefox_l10n_faqs.html#how-can-i-test-a-different-locale
[firefox preferences]: https://bugzilla.mozilla.org/show_bug.cgi?id=1415730
[firefox.com]: https://www.firefox.com/en-US/download/all/desktop-nightly/
[fluent]: https://projectfluent.org/
[fluent syntax guide]: https://projectfluent.org/fluent/guide/
[icu]: http://site.icu-project.org/
[mozilla localization best practices for developers]: https://mozilla-l10n.github.io/documentation/localization/dev_best_practices.html
[mozregression]: https://mozilla.github.io/mozregression/
[pontoon]: https://pontoon.mozilla.org/
[projectfluent good practices for developers]: https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers
[system add-ons]: https://bugzilla.mozilla.org/show_bug.cgi?id=1425104
[the ftp listing for langpacks]: https://ftp.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/linux-x86_64/xpi/
[unicode]: https://www.unicode.org/
[unprivileged contexts]: https://bugzilla.mozilla.org/show_bug.cgi?id=1407418