[fix] get correct locale with country from browser

Some of our interface locales include uppercase country codes,
which are separated by `_` instead of the more common `-`.
Also, a browser's `Accept-Language` header could be in lowercase.

This commit attempts to normalize those cases so a browser's
language+country codes can better match with our locales.

This solution assumes that our UI locales have nothing more than
language and optionally country. If we ever add a script specific
locale like `zh-Hant-TW` this would have to change to accomodate
that, but the idea would be pretty much the same as this fix.