Fix undefined var in mf-translator.js

M.settings.supportsLocalStorage moved to M.supportsLocalStorage, which
causes, that langMap isn't loaded on startup. When the user clicks a lang
link in LanguageOverlay, we try to acces an undefined var as an array. Fix
this by using supportsLocalStorage and check dependencies before do
anything.

Added a test for this case, too.

Caused by: If2e745f7bc7bb6bf24fb9b533f4b46dc5aad348b

Bug: 71920
Change-Id: Id0c89284ddb0a2561098c762352914b386d354cd
Merge "Fix undefined var in mf-translator.js"