Not sure it is an answer... let's try!
If you search to learn a new language just to enhance your efficiency to do research, I think you're loosing your time. Even in countries that had a strong tradition of writing in their own language (France, Russia), most publications are in English nowadays. Important books in Russian are quite quickly translated into English. The material contained in old articles can be found in more recent work (in my specific topic, everybody quotes a certain article in German by Hopf, by I guess few people read it). Except for history or philosophy of mathematics, the mathematical benefit of reading the original paper is very low. But learning a new language is time-consuming! If you want to be an efficient article-reader, use that time to read maths in a language you already know.
The only reason you should learn a new language is the same as non-mathematicians: learn it because you are genuinely interested in it! Of course, one interest could be being able to read the work of your favorite mathematician in its original form, but don't expect a huge impact on your career. At least, learning a new language may ease your collaboration with foreign coworkers, but it is kind of an indirect impact.