It's not a terrible idea to have a lingua franca--a tongue that most of the people in the world can at least get translated relatively easily. Here and now, that language is English, with French rather close behind.

Given the history you've outlined in your world, so long as a map was made by a Trajev or a Trajan-speaker, it would likely be labeled in Trajan. Of course, if the map is made for the benefit of a local populace, it would make more sense for it to be labeled with the local language. So I guess the thing to think about is who made the map and why? That should tell you what language(s) should be used for labeling.

For the sake of simplicity as a world-builder, I approve of your current approach.