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If the language is indicated by tags, they should be special tags just for languages. From UI perspective that might be easiest to do: just let users put an extra tag for the language in the tag box.

To differentiate language tag from normal tags, something like doublehashtag could be used (like ##eng).

Precisely. If we were then allowed to automatically add this tag to our posts and to exclude posts with other language tags, then the problem would pretty much be solved.

Something for @good-karma of eSteem , @jesta of beta.chainbb.com , the @zappl team and @fabien of Busy.org to consider.