English Language Teacher

Curently I am studying at university to be an English teacher.
I was thinking about picking another language?
What is the best foreign language to learn for an English teacher and why?
Honestly, I want to learn a foreign language that can support my future career.

Certainly Latin. English is largely based on Latin. Believe me, you'll go far with it.

It depends where you are, but if you are in America, and Spanish is probably the best.

Latin because many English words have Latin roots

Spanish as you have ESL students or Latin for help understanding the roots of English.

You should learn Arabic if you live in Indonesia
(If I'm not mistaken, is not a large Muslim population in Indonesia?)

I want to learn an easy language like Italian, then move on to difficult languages like Arabic