Well, really just one: companion.

In English, the term “companion” has, I think, a rather different connotation than the actual relationship between missionaries. “Companion” suggests living together, but not so much working together, which, to my mind, is the more important aspect of the relationship. It also entails an element of choice, which, of course, missionaries do not have. I just think it is somewhat distracting and that a different term could be more accurate and less, well, funny.

My suggestion is “colleague.” I wouldn’t be surprised if a translation of that term is already used in plenty of other languages. What do you think? Do you have other suggestions? Is “companion” your term of choice.