Knowing how to speak two languages is not enough to be able to interpret accurately between those two languages. People who speak two languages can process information in either, but the brain does not actually make a distinction between the languages in which the information was received. Bilingual people who switch from one language to another while speaking often do that because they cannot keep the information from one language separate from the other. Interpreters must start by training their brains to process information received from two languages separately. In the course of normal conversation, we can listen and occasionally shift our attention to something else, then come back to the conversation and pick up the thread of what is being said. When we are interpreting, we have to maintain our attention continuously focused on what we are hearing in order to understand the message clearly and render it accurately in the other language. A person who is not a professional interpreter will not have the ability to process information in two languages with the necessary speed and deliver the message accurately, as required in legal settings. People who are simply bilingual lack the proper training to perform the functions required of an interpreter, whether in a legal or any other setting, and are very likely to provide incomplete or erroneous information. Certification candidates who have not developed this essential cognitive discipline are very likely to fail the exam.
A certified [or Texas Licensed] court interpreter is one who has successfully passed tests explicitly designed to measure proficiency in court interpreting skills. Not all certification exams are equal (the NAJIT and federal exams are considered to be the most demanding) but most do test the skills of simultaneous interpreting (interpreting at the same time as the speaker is speaking, sometimes called “whisper” interpreting); consecutive interpreting (interpreting segments of speech out loud after they have been completed — this is used for interviews and testimony); and sight translation (oral interpretation of a written document), as well as proficiency in legal, general and colloquial terminology, good language skills in terms of grammar and usage, and, often, ethics. University degrees and certificates of attendance or proficiency from interpreting training events are not certifications. Possession of academic credentials does not necessarily indicate proficiency in the highly specialized skills needed to interpret in a legal setting.
Source: NAJIT.ORG