As I got ready to write this post I checked the AIIC site and I learned something. Let me tell you what it is when I get there.
Conference interpreters have three types of working languages. (You may have other languages which are not strong enough to work in.)
A is your mother tongue. Simple enough.
B is a language that is not your mother tongue but that you master. I say "master" to mean deep knowledge, many years experience and proficiency in speaking nearly as good as in your mother tongue. "Fluent," by the way, is actually a pretty low standard. It means that you are not halting in your speech and communicate well, the way I see it. I was fluent in French about five years before I ever stepped into an interpreting booth, for example. That might be a good rule of thumb. If you are fluent in a language, with five years of work you might be doing simultaneous interpreting into it. Anyway, You interpret from A into B usually starting out in consecutive and possibly moving on to simultaneous also.
C is a language that you understand nearly as well as a native speaker (for whom that language is a mother tongue) but when you speak your speed, vocabulary and ease of expression are just not sufficient as an "active" language. Active languages are the ones you speak, so A and B.
Here is what surprised me on the AIIC site. I had been told by my instructors at ESIT (Ecole Supérieure d'Interprètes et de Traducteurs, Paris) that working from C into B, at least in simultaneous, was too difficult, and sort of taboo, because one is managing two foreign languages (neither is one's mother tongue). But on the AIIC site we read, "an interpreter can work into this language [B} from one or several of their other working languages, but may prefer to do so in only one mode of interpretation, either consecutive or simultaneous (often in 'consecutive' because it's not so fast). It is also considered an active language for the interpreter." So AIIC has taken the position that it is not best practice but rather an interpreter's preference not to work from C to B.
Check out the AIIC text on this using the botton below.