Language selection for missions/communities

[MorpheusXAUT]

Since not everyone is interested in joining missions with a different language than their mother tongue, it might be useful to provide a setting to define a mission's language(s).
Mission creators could thus restrict (or just suggest?) the language of players in their missions.
Users could also filter for missions in a specific language.

A community could also have a language setting to a) provide a default for mission editors or b) restrict the users that can apply to their community.

Users would obviously also need a language setting associated with their profile - preferrably just a mirror of their selected locale in the frontend.

Missions and communities should support multiple language selections at once as well as an all language setting.

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Tasks

• Add language setting to missions
• Add language setting to communities
• Persist user's language choices from frontend to backend
• Add language filters to mission endpoints
• Add language filters to community endpoints