> Unless you have a specific goal in mind for learning a new language, I'd suggest Chinese. The language itself is by far much different than English (comparing with with all the language you listed above)
I'm not sure I agree that Mandarin/Cantonese are "far more different" from English than Japanese is. They are tonal languages, but AFAIK the grammatical structure of both are more similar to English than Japanese is. Japanese is rather fascinating from a linguistic viewpoint, largely because of the long periods of isolation from other languages.
That said, if one were to learn only one language, with an eye for the future, I think one could make worse choices than Chinese.
I'm not sure I agree that Mandarin/Cantonese are "far more different" from English than Japanese is. They are tonal languages, but AFAIK the grammatical structure of both are more similar to English than Japanese is. Japanese is rather fascinating from a linguistic viewpoint, largely because of the long periods of isolation from other languages.
That said, if one were to learn only one language, with an eye for the future, I think one could make worse choices than Chinese.