Did you know that speaking from a neuro-science perspective, it is easier to learn 7 languages than it is to learn 2 or 3?
Since you already have a grounding in Italian and Spanish, why not learn the other two romance languages, French and Romanian? If you want to learn another language for business reasons, I recommend Japanese. Between Russian, Chinese, and Arabic, I would probably recommend Chinese (Mandarin not Cantonese).
I don't know any languages really, but my mother taught me from very young to learn etymology (the study of the origin of words). So though I don't speak Italian (for instance) I can look at an Italian text and more or less infer the meaning.
If you're interested in etymology, I would recommend Latin.
I think most of the romance languages are useless, except for Spanish right now. That's interesting about the neuro-science perspective. I wonder why that is..But Zachor, you're right Chinese would be useful. But is there any value to learning Arabic? They say there are 3 million arabic speakers. It's good to know what your enemies are up to. Also, it's a long shot, but maybe we can convince them on how horrible and homocidal islam is if we speak their language.
I'm not sure why you think the romance languages are useless. The reason that it is easier to learn 7 languages rather than 2 or 3, has to do with something called schema theory. This is a fairly complex subject, so let me simplify it, (otherwise I will be typing all night!) I won't talk about what actually happens in the brain, with neural networks and so-on, though I would be happy to discuss it if you wish. Basically, it comes down to this:
Nothing that we learn is a self contained, isolated subject. If it were, if we were dealing with something completely foreign (and I mean completely), we would only ever understand it in an abstract sense. Now this is not actually a bad thing, by the way. When scientists discuss things such as black holes or branes or string theory even, they do not have the mental capacity to truly "visualize" these things. So their language of discussion is mathematics, which is an entirely abstract system.
When we learn anything as children, we relate them to things we already know. If you are 7 years old, and you know how to sing a song, you can understand dancing as moving to a song, so you relate the two. When you relate enough topics together, they form what are called schematic networks, that is a relationship of neurological processes that tend to work together.
The theory about 7 languages is that if you learn 7 languages all at once, you will develop a very sophisticated system of schematic networks, and this affects other schematic networks also. Furthermore, other neurological processes such as memory and emotion will also be hightened. Thus you will have a more well rounded comprehension of these languages that are not at all abstract.
I hope this helps.