allow to set labels in languages other than the actively translated languages

[maxlath]

At the moment, when editing an entities' label, the client only allows to select a language from the list of actively translated languages, which is more restrictive than what the server accepts: for the server, valid languages are all languages known by wikidata-lang (which causes another problem https://github.com/inventaire/inventaire/issues/198)

[maxlath]

To be able to transfer entities to Wikidata, the possible label languages must be limited to languages with a Wikimedia language code (see the languages list in the Wikidata API documentation)