implement language fallback

[hobofan]

Right now we are pretty quick to display a <No translation available> placeholder.

One problem is, that Wikidata seems to be inconsistent when in comes to language specifity.
E.g. some items have a label for en-gb but not en, and the other way around. There we should try the more general en if en-gb is not present.

This might also include specifiable fallback languages, so we can do fallbacks like en-gb -> en -> de -> no translation