The accepted answer on the question Other apes with gained intelligence? says about Maurice (Orangutan):
This big ape did not gain intelligence from treatment but he knows sign language because he was in circus in the past.
In the movie War for the Planet of the Apes it's seen that all other apes are speaking in sign language to one another, considering they may have learned it from Maurice. While Caesar speaks to apes in English they seem to understand, but they reply in sign language.
Caesar speaks in English almost every time, he could have taught the other apes English other than sign language. The wiki says that the third movie takes place two years after the second one. Why didn't Caesar teach them English?