This is a French question, but I'll answer in English: I'm a penultimate-year university student in languages and the two big Western European ones are French and German, those two being the industrial powers in Europe along with the UK. A large amount of professionals in Europe speak English well, but there's a lack of English native speakers such as ourselves. Russian is also valuable due to its rarity as a subject of study (for example, there are only 150 Russian graduates in the UK every year - thanks, Cold War!) and the interesting dynamic of EU-Russia relations. In short, study a language cos you like it, not cos it's "useful"! :)

I agree with Lewis, it makes much more sense to learn a language that you're interested in. That way you will have enough motivation to succeed in the long term. Nevertheless, I liked the following article and it might be useful in case you have difficulty choosing between several languages: http://www.smartplanet.com/blog/bulletin/the-most-important-languages-to-learn/