German.
Unless that's your native language, in which case that hardly counts as "foreign." Heh. Seriously 'tho, this requirement is outdated. Maybe in the 1960's, many important documents weren't yet translated, particularly from German. However, a few decades of requiring students to "take a foreign language for a science degree" has resulted in every scientific paper, no matter how trivial, being translated almost immediately from every language into every other. So default to German, or take an easier one, or ask your academic adviser and do what they say, or take an obscure one to challenge yourself, or take an ethnically appropriate one -- do you have a Russian or Japanese great-grandmother? No matter what, it will hardly matter in the future.