Preferences should support changing the language for the primary UI and if it is not possible to update the current windows, Firefox should prompt or suggest a restart. Bug 1344786 comment #3 has a use case not supported in webextensions. Preferences (talking about UI, not about:config) do not support changing the locale. Additionally, when the pref is changed via about:config, the UI in the current window does not refresh.
Bug 377881 is about supporting the switch of the UI language directly in the Fx UI, not only via about:config. About the possibility to let to WebExtensions to do it, I didn't find a related bug. I had planned to open a bug request for it, I will do it later.
bug 1425941 has traction now, resolving as DUPE.
Status: NEW → RESOLVED
Last Resolved: 5 months ago
Resolution: --- → DUPLICATE
Duplicate of bug: 1425941
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