You didn't tell us where you live and what is your first language. Since you write here I'm going to assume you already know English.
Generally:
English is essential in today's international MD RA environment. I have no reason to think this is going to change dramatically in the foreseeable future. Other than that I don't see a single language that is clearly a priority, regardless of location and intentions. However --
You might want to learn Spanish if your plans include working for US companies. I heard that there are now more people in the USA whose 1st language is Spanish than English. Plus there's an added bonus of covering most of South and Central America, Mexico, Spain, and some more countries.
IMO Chinese (Mandarin) is going to be more and more useful in the future, not just for MD RA. But it's quite difficult to learn.