Bug 1479013 shows both the incoming locale and current locale when switching languages from about:preferences with a custom bundle instead of relying on the usual `document.l10n` or fluent-dom behavior. It makes explicit calls to `formatValue` that we haven't used for about:welcome, so it would be a bit more work to start supporting that.
Bug 1778792 Comment 5 Edit History
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Bug 1479013 shows both the incoming locale and current locale when switching languages from about:preferences with a custom bundle instead of relying on the usual `document.l10n` or fluent-dom behavior. It makes explicit calls to `formatValue` that we haven't used for about:welcome, so it would be a bit more work to start supporting that. https://searchfox.org/mozilla-central/rev/4f2984be127d2e7c788cf1848d63dca63022beec/browser/components/preferences/main.js#1062-1072