true story, i just came from another country and i don't speak fluent English, so i m lost in all my classes, i have to spend hours with a dictionary so i can translate the problems in the books.
My mother tongue is French and I moved in Argentina about 2 years ago... I had no major difficulties in learning and understanding the stuff in Spanish. I don't translate anything, but rather I love to learn in French, Spanish and English (since a few), thanks to wikipedia. So when you don't know how is called a theorem, or a concept, just check it out on wikipedia.
But if you have difficulties with the language in everyday life, I strongly suggest you to take English courses adjusted to your current level. There's nothing better than that : ask all the questions/doubts you have... You'll progress spectaculary.
Anyway, a decent Spanish dictionary would be the dictionary of the Real Academia Española. I don't own it myself (too expensive), but I believe with no doubt that it's the best one in Spanish.