Show user which languages have support

[Meesch]

Currently, when uploading their own corpus, users can select any language that has an iso code, but not all those languages have stemming or stopword functionality in ElasticSearch. We should make it possible for users to see which languages have these functionalities and which don't. Potentially we can even allow them to upload their own stopword lists? Should not be too difficult to implement, as we are already using local stopword lists for several languages.

[lukavdplas]

I would prefer not to have an upload feature, actually. If users want to submit new lists, we can add them to this repository. That would be more collaborative and transparent to users.