I would say either Latin or French. Latin because it really helps you understand English words that you don't know, and even Spanish and French ones. I say French because (I'm assuming that you live in the US, sorry if I'm wrong) a lot of people up north (Ohio, Michigan, etc.) are French. (WARNING! History lesson!) Because at one point, Quebec was a French territory, and then the British took it over, and then they won independence. But anyways, a lot of people wanted to stay loyal to their culture and did not want to be under British control, so they moved to America. Some of their descendants still speak French. And obviously people still move from there to the US.
If you already know Spanish it will be easy for you to learn Portuguese because they are very, very similar. Brazil is one of the top five economies that are growing the most and is attracting a lot of investors. If you speak Portuguese companies will pay big bucks to hire you.
In my opinion, it is best to know a language from the roman , and a language from the german family of language. So german and latin are good. Latin- used in various ways when it comes to science and all knowledges, and german- a language that comes quite handy, seem like a good choice. And this way you'll be able to connect several languages that are similar and belong to the same family.
Latin because you need it to be a scientist or a doctor all those species names and prescriptions are latin also it helps with the roots of words most come latin such as: science (to know) expert (to test or try) even etc is latin (et cetera) ps means Post scriptum (after writing not personal secret) but of course im a latin student take it from me you should do latin I recommend ecce romani books if you cannot get a teacher.