The value of interpreter certification, moreover, depends on the validity of the testing methodology and other certification requirements. Some states use court interpreter examinations that are of dubious validity. Federal certification is usually a good indicator of competence. State certification may or may not be a reliable indicator, depending on the state in question. It is a good sign if a translator you are considering is accredited by the American Translators Association in the appropriate language combination.
Reliable certification is one of the most important issues of the day.
First, know your languages. Full bilingual proficiency, ample vocabulary, and a knowledge of standard grammar are prerequisite. Court interpreter training normally focuses on specialized vocabulary and interpreting skills. There are occasional short-term seminars and workshops around the country. One indispensable self-training technique is to go to court and observe the proceedings.