Thanks to a post on ads-l, I saw this story from the LA Times. It lays out the difficulties of a California court in finding an interpreter for a Mixe speaker who was on trial.
It's worth reading, but note that this is a pervasive problem and one that has been dealt with horribly many times in the past. The most egregious case I know of is this one, of Santiago Ventura, a Mixtec speaker convicted of murder without the benefit of interpreters who spoke his language. He spent years in jail before it was sorted out. As it happens, both these languages are spoken in Oaxaca and a little info on the major languages of the region is here, and it's the source of this map, although as the Times story makes