@mikedyang already considered for a multi-language community:
- separate discourse instances
- category for every language
- tag for every language
I suggest a 4th option: Groups for every language.
Discourse know allows open access groups: users can join and leave freely.
One could give access certain categories only for members of certain groups.
Advantage:
Pros
- digest does not contain other languages
Cons
- more content is private and cannot be oneboxed from other places.