Mastodon relies on users setting the language their post is in manually, so if someone posts in two languages and forgets to switch between them, they don’t get filtered out. I know there are some other pieces of software that switch it automatically, I’m fairly sure Calckey automatically recognized the language you were writing in.
again, if the person who is making the post doesn’t change the setting, it won’t get filtered. if they type a message in German but the post’s language option is set to English (which I think is the default on some major instances), it won’t get filtered out.
you can usually check what their post’s setting is by starting writing a reply to them, as the language option of your post will switch to the one they post in.