typeahead i18n/l10n support

[arsylum]

Currently the inputs only provide typeahead suggestions for English labels since they are loaded with the json. Do we want to support typeahead in different languages?

Fetching all the entity labels for a language is on the expensive side but maybe the MediaWiki API wbsearchentities action could be utilized?

[mkroetzsch]

I think wbsearchentities might only work for using all entities, as required in issue #37. One could combine the feature with our local record of all classes to filter suggestions.

An alternative could be to use SPARQL instead, which might also be useful for other similar features (when selecting possible values of a property etc.). However, SPARQL may not be quite fast enough for this usage.

For properties, it could be possible to fetch the labels of all of them in a different language when needed. This could work fairly well with SPARQL. But for classes, this is probably really too much.