U.S. Citizenship and Immigration Services (USCIS) does not provide foreign language interpreters at adjustment of status (green card) interviews in the United States. A few of their officers speak Spanish or other languages, but you can’t count on getting a bilingual officer, nor can you request one.
If you are not comfortable speaking in English, you will need to bring a friend or hire an interpreter to attend your adjustment of status interview. Even if your U.S. petitioner -- that is, the spouse, parent, or other party who is sponsoring you -- is capable of interpreting for you, the USCIS interviewing officer may not allow it. That's particularly true in a marriage-based case, because it reduces his or her ability to compare your two answers and detect marriage frauds.
The interpreter does not have to be a trained or licensed professional. The only requirements are that he or she be over the age of 18 and fluent in both your language and in English. Some officers also require that the interpreter be a legal resident or citizen of the United States (of course, if they are here unlawfully, they would be foolish to walk into a USCIS office).
However, interpreting is no easy task, even for someone fluent in two or more languages. Whoever you bring should practice with you beforehand, going over your application and getting familiar with the vocabulary used there. You should also remember, during the interview, not to give any overly long answers. If you don't give the interpreter a chance to step in regularly, he or she is likely to forget your full answer, and some of it may get lost -- leading to confusion or misunderstandings with the USCIS officer.