Talking about your general language learning exploits is okay, I think. These are basically a support community for Japanese learners to relax in, and there are bound to be people who are studying more than one language. It can have relevance in regards to the general language learning process, too. Just so long as it doesn't become an overwhelming staple in the threads, to the point where it's basically DJMGRST (and don't you dare get one of those letters wrong, shitlord): Daily Japanese, Mandarin, German, Russian, and Spanish Thread. In that case, it would be time to ban those particular topics.
I'd say that actually posting in Chinese is kinda passing the limit in contexts where the contents of what's being talked about aren't obvious without a shred of Chinese knowledge, though. Entering a community where the residents speak in a particular language and posting in foreign tongues that most people in the community don't understand is intrusive and exclusionary towards the members of that community. It's only natural that they would get irritated.