Enhancement: Added language option to the app provider

We've added a language option to the app provider which will in the
end be passed to the app a user opens so that the web ui is displayed in the users language.

https://github.com/owncloud/ocis/pull/4399
https://github.com/cs3org/reva/pull/3156
https://github.com/owncloud/ocis/issues/4367
