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I am learning Japanese but while I enjoy it, I wouldn't recommend it unless you have a use for it as there are a bunch of additional difficulties in learning it than in learning a European language.
Chinese would be good from a work perspective because it is an important language for business but the Chinese are all awful at English and there is plenty of demand for quality translators (especially as you know a few other languages as well). Another advantage is that their literature and TV doesn't get translated anywhere near as frequently as European equivalents do. On the other hand, it is another extremely hard language to learn so might not be what you are looking for.
Otherwise something like German or French would probably be best as their respective countries are still the most important countries in continental Europe (not counting Russia) and have plenty of native speakers (although they are relatively knowledgeable of English so its not really necessary to know the language unless you plan to move there).
In general it just depends on what you are learning language for as the answer changes wildly depending on if you are interested in making money, being able to communicate with more people, or due to having an interest in the culture/literature.
