language: Use item 'fallbackSequence' instead of duplicating logic

language: Use item 'fallbackSequence' instead of duplicating logic

The 'fallbackSequence' is exactly generated for this purpose. It
is equal to the value of 'fallback' after splitting, trimming
and ensuring 'en' is present. See LocalisationCache::recache().

Also simplify returning of the first array index by returning it
directly instead of modifying the array first.

Due to an inconsistency between how LocalisationCache and Language classes
treat the fallback sequence differently, we have to manually fallback
to 'en' in case of unknown language codes. This is because otherwise
Language::factory() will throw an exception causing tests to fail.
We should investigate whether this is desirable or not, but keeping
existing behaviour for now and documenting it.

Change-Id: I9c1d51b59aabebf5a31f38205304bb8cc22dcd8c