For most Americans a foreign language is not necessary. So, because it is required and college courses are expensive you should probably take something that you will be able to use. For instance if you are going into medicine, chemistry, biology or a number of other degrees Latin would be a good choice. If you are interested in Anime and Manga Japanese is a good option.
The only language that has a direct application to STEM is Latin and its application is largely in naming schemes. The other use for it is to communicate with other sciences, however the Lingua Franca of science is English.
At this point all STEM degrees require the student to take two semesters of a Foreign Language. Fortunately these courses can be completed at a community college. Not all colleges offer all Foreign Languages, but because most state community college systems are connected and online you can find the one you are looking for.
Which course you take is not a major life decision. Pick one that you will enjoy and have fun. Also, try to take these courses when you have the time to commit to it.
Most common Languages are: Spanish, Mandarin, French, German, and Japanese.
There are other less common options and it is highly recommended that you take whichever one interests you the most.