How to prepare for the French language exam as part of PhD program?
Yes. I took both German and French. I was already fairly comfortable with French from having studied it in high school and in university, so I'll write about German instead. I prepared for my German language exam by going on to the GDZ server (a program to retroactively digitise documents, including a lot of classical mathematics papers) and downloading some papers in mathematical analysis (my field, sort of) from around the 1950s to 60s (when still a lot of the papers were published in German). I then went to the library and borrowed the copy of Hyman's German-English Mathematics Dictionary (for the nouns) and a copy of a normal German-English dictionary (for the verbs) and sat myself down and started translating word by word. After about a month I translated about 30 pages or so of math papers (and at the same time learned about uniform spaces), and proceeded to take my exam. 
Not much. I have absolutely no conversational German, and only very rarely do I have to look something up in German that I cannot find a write-up in another language. (So far twice only, one a slightly obscure result in algebraic lattices that the only reference I could find is to a paper in the 30s.) (French on the other hand has been useful; mostly because the abovementioned "write-up in another language" often turned out to be in French.) Given how much I've found French to be useful for me, I'm sure that if I had actually properly learned the language, German would be just about as useful. Conversely, if you don't really learn the foreign language, you will probably be able to get by through other means in this day and age. 
The canonical place to find French papers to read is NUMDAM. Of course, for practice, you can also check out Bourbaki from the library; it may be slightly less intimidating to start working first on the language by reading something whose mathematics at least you are familiar with.