You can change the language for the entire community from Admin Panel> Settings> Language. It changes the language for the entire community, all the buttons, notification messages, ...
However, still you can use another languages keyboard to post your contents in any language. Just like Fb or Instagram.
We have "Translation" app that helps you translate all the posts form any other languages to English, which can help you understand your user's posts from any language.
I would recommend to create different "Groups' for each language. For example, create a group for "German", where all users are posting contents in German. That can be the place for your German users.
However, we have the multilingual community available though the extra instances, which is an option for our Premium and Enterprise clients only. You can learn more about it here71.