Interpretation is a demanding, meticulous, highly specialized and exciting field. It requires great mental stamina, flexibility, extensive general knowledge, and native fluency in the languages interpreted.

Court interpretation requires all of the above, plus an intricate knowledge of the justice system, its terminology, and equivalent terms in the target language. Court interpreters adhere to a strict codes of ethics for interpretation in a legal setting, and must have the ability to work well under pressure.

Who uses court interpreters? Courts, attorneys, law firms, insurance companies, law enforcement, and others who need to know that what they and their interlocutors say is being transmitted as faithfully and as accurately as possible. Court interpreters are used for depositions, attorney-client conferences, criminal, civil, or juvenile proceedings.

Court interpretation is a developing field. Many languages still have no court certification, but interpreters may hold other qualifications that guarantee their competence as court interpreters. The more common languages for which interpreters are used in the United States judiciary are Spanish, Haitian Creole, Navajo (for all of which there exists federal certification), Arabic, Mandarin, Farsi, Hmong, ASL (American Sign Language), etc.

SOME FAQS:

Why should I pay for a certified court interpreter? There's someone cheaper all the local attorneys use, and "she's really good.”  How do you know that an interpreter is “really good” unless you are fluent in both languages, and versed in the protocol of legal interpretation? 

What does using under-qualified “interpreters” say about you as an attorney?

In Spanish, there’s a saying, “El dinero del pobre va dos veces a la tienda” (lit. "the poor man’s money goes twice to the shop," or "you get what you pay for"). 

By using a non-certified or non-judicially-qualified interpreter, you are taking a big chance with the integrity of your client’s defense. You are handing the other side an opportunity to appeal based on faulty interpretation. It can even be dangerous, since some unscrupulous people act as “interpreters” in order to gain information about rival drug gangs, etc. Certified interpreters have passed criminal background checks. They are trained to divulge conflicts of interest, and keep confidentiality. 

Why can't I interpret for my client? I'm bilingual.  
Among other things, because you have a conflict of interests, but also: are you certified as an interpreter? Most people who are simply conversant in two languages use incorrect terminology and false cognates when interpreting, because they are not professional interpreters. Take a look at the case of poor Willie Ramírez,  An amateur interpreter cost him the use of his legs, and premature death, and cost the hospital that misdiagnosed hi, $71,000,000. 

Can using an under-qualified interpreter create appeal issues? 
Ask the attorneys in United States v. Bailon-Santana, 429 F.3d 1258 (9th Cir. Dec. 6, 2005) (No. 04-50079).  Oops!

My client's first language is a “dialect” but he can communicate OK in Spanish.”

A dialect of what? Mayan? Indigenous American languages are as similar to Spanish as French is to Korean: do not assume that your Latin-American client is fluent in Spanish. Also, as a result of historic political oppression,  at times, indigenous Latin-Americans tend not to “rock the boat” by requesting language assistance, or even by asking for clarification if they don't understand something. To them, it may seem polite and correct not to do so, especially when dealing with the authorities. 

Language fluency must be assessed, and then a different interpreter located if needed.

Sometimes, in the case of exotic languages, relay interpreting is required:

Kanjobal speaker < > Kanjobal/Spanish interp. < > Spanish/English interp. < Court