Hello Canapin
In my opinion
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Use group for restrict the Other language category and add a notice banner topic, a button or some notification for user. If they want to activate that category, join the group. (users can choose which content they want to see)
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There is a really nice plugin from Pavilion which is use tags. I used it one of my site. There is an option Content Language. It create a new field in composer which is Language(s) When users create new topic they have to add the content language(s) and this tags the topic to the correct language(s). Users can select in settings/interface which languages interest and only those topics appears on the forum. But this one maybe not for forums which contains million topics because all topics have to have Language tags.