Optimus Prime speaks to the Dinobots in English, compelling them to join in his fight. However, the only sound we ever hear from the Dinobots is their animal roars. Are they incapable of speaking/communicating with another Transformer?
We don't know
So far, the AOE movie is the only canonical appearance of the Dinobots in the Bayverse movies. However, the video below (taken from this forum) surfaced a few months ago.
In it, a voice artist is shown to be voicing Grimlock (leader of the Dinobots) speaking in English for a yet unreleased toy. How canonical you want to take this is really up in the air, as the Bayverse toys have been known to introduce characters and concepts not in the movies themselves.
In the cartoon G1, the Dinobots were created by the Autobots to be as strong as dinosaurs. "Of course, the Dinobots have simple brains, just like real dinosaurs." So, they could speak, but they spoke like cavemen.