It speaks more to ethical considerations for the client, the speakers, and the interpreter. You know, in interpreting we sort of have three categories of language. An interpreter has an A language, a B language, and a C language. In the C language, which is the least mastered by the interpreter, they would only interpret passably from C into their A. In A and B the interpreters would work both ways.
If you were to ask me as an interpreter to interpret a language I don't know, it would put me in an ethically difficult situation because I don't understand what I'm saying. The speaker doesn't necessarily have faith in me to render it. In the equation there's not enough certainty to understand the language properly.