The previous version of this allowed for rendering languages even if no
autonym was present. This can theoretically work for the LanguageOverlay
which shows language links for titles; but since titles are not rendered
in the LanguageInfoOverlay, the layout ends up looking broken if
autonyms are missing as well.

This change also filters out legacy language codes.

Finally a comment in LanguageInfoOverlay has been removed, since it was
partially incorrect: the CLDR extension provides the autonyms used by
the languageinfo API, and when this is present these autonyms can be
used for search / filtering in the language info overlay.

Follows-up: Ic0f1613e8fd27788f67ed6235386491f0fe077c3
Change-Id: I5ae77a89c8d448641082002e09452011bf2516dd