One of the reasons for creating groups is to serve this purpose. If everyone would be opening threads on the general forum with their mother tongue language then we would really have a problem understanding each other.

Having individual groups helped organizing the website and give people the chance to discuss their issues or interests in a way that won't disturb the general forum or other people.

If I would be for example opening always threads talking about fishing on the main forum, then it would be kind of disturbing to other people [in addition to the smelly threads :)] and will confuse them.

People find it easier to communicate in their own language, if you can understand the language and comment on the threads then that would be great. If you can't then it is ok, the people their would appreciate and understand that you were trying to help somehow but could not because of the language barrier.
Cheers brother and have fun ...

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Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment. (Gautama Buddha)