Ya that sounds reasonable. And it may also depends on the place where you are, if you visit Shanghai, maybe some people won't understand mandarin even by a native Chinese person, they have their own dialect, especially old people. However, nowadays mandarin is propagated quite widely, almost young people understand it within China or in Malaysia, Singapore, etc. By the way, if you learn Chinese well, Japanese might be somehow easy already, at the same time, if you learn Japanese already, Chinese would also be easy. Languages belonging to same linguistic family are similar to each other.