Add i18n-getAvailableLanguages() proposal

Author: carlosjeurissen

proposals/i18n-get-available-languages.md

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+# Proposal: `i18n.getAvailableLanguages()``
+
+**Summary**
+
+An API which allows extensions to get a list of BCP47 language tags containing
+the languages the browser can use and the extension provides resource files for.
+
+**Document Metadata**
+
+**Author:** carlosjeurissen
+
+**Sponsoring Browser:** TBD
+
+**Contributors:** hanguokai
+
+**Created:** 2025-01-23
+
+**Related Issues:** https://github.com/w3c/webextensions/issues/258 and https://github.com/w3c/webextensions/issues/274
+
+## Motivation
+
+### Objective
+
+Currently extensions can not easily get a list of supported languages without
+hardcoding them first or need to rely on fetch try and error.
+
+#### Use Cases
+
+Allow to list the languages for extension settings language pickers.
+
+Use the list of supported languages to match a specific website when applying
+a content script.
+
+Have this list independent of distribution (As some extension stores disallow
+certain language tags).
+
+Allow third parties to create polyfills for `i18n.withLanguage()` and
+`i18n.setLanguage()`.
+
+### Known Consumers
+
+Extensions who currently have to maintain a manually currated list of supported
+languages to offer content script language matching and/or allowing to change
+the extension language in the extension settings.
+
+An example of content-script matching is "Category Tabs for Google Keep", which
+needs to match it's language with the page in question to properly integrate.
+
+A simple naive implementation would be:
+```js
+const [pageLanguage, supportedLanguages] = await Promise.all([
+  tabs.detectLanguage(),
+  i18n.getAvailableLanguages(),
+]);
+const resourceLanguage = supportedLanguages.includes(pageLanguage);
+const resourceFile = chrome.runtime.getURL(`_locales/${resourceLanguage}/messages.json`);
+const messages = await fetch(resourceFile);
+```
+
+Future consumers of future related APIs like the `i18n.withLanguage()` and
+`i18n.setLanguage()` APIs.
+
+## Specification
+
+### Schema
+
+```typescript
+namespace i18n {
+  function getAvailableLanguages(): Promise<string[]>;
+}
+```
+
+This API should also be exposed in content scripts.
+
+### Behavior
+
+Given an extension with the following resource files:
+`_locales/en/messages.json`, `_locales/es/messages.json` and
+`_locales/es_419/messages.json`.
+
+The `i18n.getAvailableLanguages()` method would return a promise which resolves
+with the following array: `['en', 'es', 'es-419']`.
+
+If no resource files are specified, and/or `default_locale` in the manifest is
+missing, the promise will reject with an error stating extension localisation
+is required for this method to work properly.
+
+If a browser is unable to offer a specific language tag in `i18n.withLanguage()`
+or `i18n.setLanguage()`, for example, because it does not understand the tag
+(think about `_locales/n/messages.json`), or if a directory has no messages.json
+file, it should not include said language tag. This does not mean a browser
+should only return languages the browser UI itself supports. An extension could
+offer additional languages outside the set of languages used for its own browser
+UI. In that case browsers can offer support for these languages and preferably
+should support and return them.
+
+### New Permissions
+
+No new permissions are needed.
+
+### Manifest File Changes
+
+No manifest file changes are required.
+
+## Security and Privacy
+
+### Exposed Sensitive Data
+
+The returned information could in theory already be known to the extension. This
+information is not sensitive.
+
+### Abuse Mitigations
+
+As this does not expose sensitive data, there is no abuse potential.
+
+### Additional Security Considerations
+
+Exposing the list of locales an extension has resource files for is unlikely to
+introduce any new security risks.
+
+## Alternatives
+
+### Existing Workarounds
+
+Extensions could hard-code the supported locale tags. This is however error
+prone as the list may be outdated. Some extensions may also reduce the list
+of included resource files for compatibility with specific extension stores. See
+https://forum.whale.naver.com/topic/39749/
+
+### Open Web API
+
+I18n resources are currently extension only. While there are talks about
+open web dom localization (See https://github.com/mozilla/explainers/blob/main/dom-localization.md)
+They are far from ready and their handling of resources will be different from
+current web extensions.
+
+## Implementation Notes
+
+Since this information will not change during the lifecycle of an extension. It
+can easily be cached on installation/update.
+
+The locale tags need to be turned into a valid bcp47 tag. For example, for the
+resource `_locales/en_US`, the bcp47 tag `en-US` should be returned.
+
+## Future Work
+
+Allow to use `i18n.getMessage` with a different language. See https://github.com/w3c/webextensions/issues/274
+Allow to use change the language of extensions. See: https://github.com/w3c/webextensions/issues/258#issue-1344495962 and https://github.com/w3c/webextensions/pull/641