Define which languages explicitly fallback to 'en'

In the message store, all messages fall through to English,
but only a few languages should actually explicitly fallback
to English (English variants and dialects).

These new explicit fallbacks are used by ResourceLoaderImageModule,
and this change doesn't affect the message fall through system.

Bug: T203350
Change-Id: I6b68a17f4d69341bccdae748727b5133a600d8bc