It seems to me, that the search functionality of flarum only searches in the body of posts, not in the title of a discussion. That's independent from the language.
E.g. search for Discussion.moe, you will not find this discussion:
Discussion.moe // General, Tech, Gaming // Online Community!
In the case of the original poster, a search for 这是正文测试 (in the body of the original post) works, but not a search for 中文搜索测试 (in the title). I think this behaviour could be covered by an extension, but in my opinion should already be in the core software.
