About choosing the language to study, what do your impulses tell you? Are you limited to choice only among French, German, or Russian? Is any significant work in Physics done or written in any of those ? (Obviously, YES). They are all great languages, as are others. A language's perceived difficulty is unimportant. German is not a difficult language to learn; Russian may not be difficult by having 'more letters' to learn, aside from being more phoneticly consistant than English.
Here are some useful hints to inspire you, maybe:
French is widespread, not in all places, but in many places. Being Latin related, it has contributed some vocabulary to English, so this will help to understand and appreciate English a bit better. It may help you in on-the-spot crossover communication with other Latin related languages, although very different from them (meaning still not mutually intelligible, but sometimes helpful anyway).
German can help you understand more about the structure and origin of English. English is a Germanic language. Not as widespread, but why let this be an obstacle?
Russian is known, spoken in a big region, otherwise not so widespread; also, has similarities to other well known languages in neighboring regions, making learning any of them possibly easier after learning Russian for a few years. Russian is not so similar to English, but really, why should this be an obstacle? So many million people already know Russian as their first or second language - why fear it if they could do it?
Why are you in a hurry to get your language requirement "out of the way"? Being able to communicate using someone else's language is a good thing, and not so tough to learn. You have already been studying harder with Physics, Engineering, and Mathematics than you will need to study for a foreign language; German, French, Russian are maybe complicated, but by being a part of human natural language, they are easier to learn (if taught well) than what you have been struggling with during your last few years.
