Who can translate a legal document?

I need my foreign birth certificate translated for immigration. I was born in Iran, but grew up in Germany, and I've applied for a green card. My wife is American. I received a letter back from USCIS that says I have to have the birth certificate translated. Are there "legal" translators? Or can anyone who can do it, allowed to? I mean is there some special certification? I tried to call the USCIS site to find out, but I gave up.