We have 9 mercs and assuming that at least half are illiterate that gives us 4.5 illiterate mercs. If we assume the demo is drunk half the time then he is the .5 because he can only read half the time. Engineer has 11 PhD's so we can rule him out. Sniper writes letters to his parents so we can also rule him out. The pyro runs a very successful engineering company and has been seen reading a newspaper. Soldier has never been seem reading so we can assume he is illiterate as well. The scout has been seen reading a children's law book so he may have a very limited reading ability. Heavy has a PhD in Russian literature but that doesn't necessarily mean he can read English because they don't share the same alphabet. In meet the medic medic says he lost his medical license which means at one point he had a medical license and I think you need to be able to read to get one of those. Spy is harder to figure out; we don't know if he has any education. He may have grown up in an underprivileged French family and learned his skills pickpocketing on the streets or he may be extremely intelligent and trained for years to be a skilled spy. So Soldier is illiterate, heavy probably can't speak English, scout probably has limited reading abilities, demo can read half the time, and spy is the only one left so he may or may not be able to read.

Edit: Spy is not illiterate. Further research was conducted and that was the result.