Not really true. Land Before Time 5 has a conversation with Chomper and his parents, and it's given subtitles. You could dismiss this as a funny little thing, except right after, Chomper pretty much translates what his parents said (by stating that his friends won't be safe around them). At the end of the movie, Chomper acts as the translator for his parents when they extend peace to the children.
Then we have an episode of LBT the TV show, where Chomper has a conversation with Thud, which is also subtitled. And in another episode, Old One shows genuine shock that Chomper can speak leaf-eater. If sharpteeth speaking leafeater is a bit common, she wouldn't have been surprised.
So Chomper is one of the exceptions, since he is bilingual.