I’m learning Japanese and I’ve had a very similar experience... although I haven’t noticed the Chinese results, which is surely a result of my slow progress! (o_0)
In my experience, the Chinese results tend to happen when the entire query is in kanji. Queries that have at least some kana generally aren't an issue. If it doesn't make sense to use kana, then I'll sometimes add "site:jp" as a workaround, since most Japanese-language sites do use the .jp TLD.