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+# Proposal: i18n.getPreferredSystemLanguages() and i18n.getSystemUILanguage()
+
+**Summary**
+
+Allows getting the language(s)/locale(s) of your operating system as a [BCP47 language tag](https://www.rfc-editor.org/bcp/bcp47.html)
+
+**Document Metadata**
+
+**Author:** [carlosjeurissen](https://github.com/carlosjeurissen)
+
+**Sponsoring Browser:** TBD
+
+**Created:** 2024-03-18
+
+**Related Issues:**
+https://github.com/w3c/webextensions/issues/252
+https://bugzilla.mozilla.org/show_bug.cgi?id=1888486
+
+## Motivation
+
+### Objective
+
+Allow extensions to display UI in the user's system locale(s), even when this is distinct from the browser's UI locale (which is restricted to locales supported by the browser).
+
+#### Use Cases
+
+Extension authors might want to obtain the locale used by the user agent's host environment in order to format values (such as numbers, dates, and so forth) or do other locale-affected operations (such as list sorting) according to expectations of the user. For example, formatting the date `2024-06-15` varies between `en-US` (6/15/2024) and `en-GB` (15/6/2024), whereas `i18n.getUILanguage` might return only `en` (English).
+
+Extension authors might want to obtain the locale used by the user agent's host environment to better match the runtime environment's localization. For example, the operating system might be running in `ff-NG` (Fulani as used in Nigeria), but the user agent might not be localized for this locale and return `en` (English) for `i18n.getUILanguage`.
+
+Extension authors might want to obtain the locale used by the user agent's host environment in order to get the locales in it's purest form. For example, the operating system might be running in `es-AR` and a browser's closest equivalent is `es-419`. Having access to `es-AR` allows extension authors to provide better translations for all users.
+
+### Known Consumers
+
+Language-related extensions and extensions wanting to match the locale of the operating system more closely independent of the locale of the browser.
+
+## Specification
+
+### Schema
+
+`i18n.getPreferredSystemLanguages()` would asynchronously return a language priority list (ordered list of [BCP47 language tag](https://www.rfc-editor.org/bcp/bcp47.html) like `i18n.getAcceptLanguages()` does right now.
+
+```json
+{
+  "name": "getPreferredSystemLanguages",
+  "type": "function",
+  "description": "Gets the preferred languages of the operating system. This is different from the languages set in the browser; to get those, use $(ref:i18n.getAcceptLanguages).",
+  "parameters": [],
+  "returns_async": {
+    "name": "callback",
+    "parameters": [
+      {"name": "languages", "type": "array", "items": {"$ref": "LanguageCode"}, "description": "Array of LanguageCode"}
+    ]
+  }
+}
+
+```
+
+`i18n.getSystemUILanguage()` would synchronously return a [BCP47 language tag](https://www.rfc-editor.org/bcp/bcp47.html) like `i18n.getUILanguage()` does right now.
+
+The returned language tag can be different from the first entry returned by getPreferredSystemLanguages as the operating system could not support all languages specified by the user for its user interface.
+
+It would follow the following signature for [i18n.json](https://chromium.googlesource.com/chromium/src/+/4299ce68496b32ba309e2f012e0db5b4b8cd478a/extensions/common/api/i18n.json):
+
+```json
+{
+  "name": "getSystemUILanguage",
+  "type": "function",
+  "nocompile": true,
+  "description": "Gets the current UI language of the Operating System. This is different from $(ref:i18n.getUILanguage) which returns the UI language of the web browser.",
+  "parameters": [],
+  "returns": {
+    "type": "string",
+    "description": "A BCP47 language tag such as en-US or pt-BR."
+  }
+}
+```
+
+### New Permissions
+
+As `browser.i18n.getUILanguage()` does not require a permission, browser.i18n.getSystemUILanguage and browser.i18n.getPreferredSystemLanguages also should not.
+
+### Manifest File Changes
+
+No new manifest fields
+
+## Security and Privacy
+
+### Exposed Sensitive Data
+
+The locale(s) of the system will be purposely exposed.
+
+## Alternatives
+
+### Existing Workarounds
+
+Currently, if an extension wants to offer locales outside not localised for by the browser, it must do so by presenting a user-facing options page. However, currently there is no way to reasonably detect a language code outside of the browser's UI language with existing APIs.
+
+### Open Web API
+
+To combat fingerprinting, this should not be an Open Web API, like browser.i18n.getUILanguage is not.