i18n Fails to detect additional languages.

In build 14971 i18n fails to detect additional languages. Issue non-repro in 14965.

If the language set in manifest is en but additional languages are available to use, The browser will always fall back to en and not use the preferred based on browser language.

Repro Steps:

Set the language on system to en-GB so the browser detects it as GB and run the attached extension on build 14965. Notice it will load GB strings (notice customise instead of customize on the about Delete repeated word screen.)

Now do the same on 14971, it falls back to default set in manifest and doesn’t load the preferred language or localised strings.