#1: Speak slowly.
Most people speak too quickly, especially when they are nervous. People are more likely to be nervous when in court or in a deposition, and this is when interpreters are trying to convey every word that you say. Therefore, if the interpreter is going to be able to repeat everything in another language, you need to speak slowly. Chances are that you are probably still speaking more quickly than you think, anyway. This rule is even more important if you are using medical terminology or other specialized terms which the interpreter is not likely to encounter very often, or for which there may not be a direct translation and which may need to be explained to convey the meaning.
#2: Read even more slowly.
People read much more quickly than they typically speak. If the document that you are reading is important (and it probably is, or you wouldn't bother to read from it), then you should slow down while reading to make sure that the words are translated.
#3: Don't go off on tangents.
If the interpreter asks you to repeat something that you have said, do just that—repeat only what you said. Do not go off on a tangent explaining the concept or background story to the interpreter. The interpreter was not asking you to repeat yourself because s/he did not understand the concept, but only because s/he did not hear or understand the words that you said.
#4: Do not interrupt or speak over another person.
An interpreter can only take interpret the words of one speaker at a time. When people talk over one another or interrupt each other, the record becomes muddled and the interpreter cannot keep up. Wait until one person has finished speaking before you speak.
#5: Answer out loud, using real words.
Do not nod or shake your head, as an interpreter cannot convey this. Try to minimize mmm-hmm, hmm-mmm, uh-huh, and huh-uh, as these are often difficult to hear, and may lack direct translations. Use "yes," "no," and other real word verbal responses.
#6: Minimize the use of slang.
Slang often cannot be directly translated, or even conveyed in a meaningful way to speakers of other languages. Minimize the use of slang, making the interpreter's job easier, and making for language easier to understand by the listener.
#7: There is a difference between a translator and an interpreter.
Did you know this? I didn't until an interpreter explained the difference. A translator changes language from one to another in written form. An interpreter changes language from one to another in verbal form.
#8: Court certified interpreters really are better.
Interpreters who have gone through the process to become court certified interpreters really do a better job. They have the skills necessary to convey meaning from language to language, and understand that the role of an interpreter simply is to repeat what is said (to the extent possible). When it is possible to use a court certified interpreter, I recommend it.
#9: Interpreters are not lawyers, however.
So we cannot assume that interpreters understand legal concepts well enough to explain them if there is no direct translation. Some interpreters have been working in the court system that they do understand, but not all will. So attorneys need to ensure that they are explaining concepts, and not just trusting or assuming that the interpreter can or will.