Add support for language variants in LanguageOverlay

Also consolidate API requests so that only one is
necessary to get langage and language variant data.

Remove now-unneeded test and expand existing tests
to test language variants.

Bug: 58988
Change-Id: I706e29d554bf09393ba27523f360e5d28f4576b8
Merge "Add support for language variants in LanguageOverlay"