This feature may be useful in other scenarios too, but my use case is a multi-lingual site. A key challenge of a multi-lingual forum is how to best configure it to allow the use of different languages while not spamming users with posts in languages they don’t understand.
At present, I think the best solution is to use language groups and show categories for each language only to members of the respective language group. Like this:
But there is a significant problem with this approach: all content (perhaps except for the default language) will be hidden from users that are not logged in.
So here is my suggestion for a feature: the problem would be solved with a category security setting “hide category from these groups”. That way all content would be visible to everyone but when you login, you only see your languages. (Positive side effect: people are encouraged to log in)