What about the voice over thingy? They could still roar and grunt but be understood like Spike was in "Through the Eyes of a Spiketail".
Also, Chomper
isn't the only Sharptooth to talk in English.

Also, the characters above
are villains. Also, the Sharp Beak from film V that grabs Ducky and the Belly Draggers from film X and the TV Series can't talk in English. Why are these two singled out?
And, as for Chomper's bilingualism, that could be quite useful to the good guys (leaf eaters) or very harmful to them, depending on his alignment when he is older.
Good Future Chomper
Chomper: (Speaking in Sharptooth) The Leaf Eaters went that way. (Points some direction)
Sharptooth: (Speaking in Sharptooth) Thanks. (Heads off with other Sharpteeth)
Gang of Five: (Coming out once the other Sharpteeth are gone) Thanks Chomper!
Chomper: No problem.
Bad Future Chomper
Chomper: (Speaking in Leaf Eater (English)) Hello, over here. I need to talk to you.
Spike Tail: Ok. (Heads toward Chomper, who is hiding on the other side of a ledge and hence can't be seen) (Comes near Chomper) What the?!
Chomper: (Speaking in Leaf Eater (English)) Glad you joined me for dinner! (Attacks Spike Tail)
Also, if Chomper gets a girl friend when he is older, it might be nice to show her at first unable to understand Leaf Eater and only hear leaf eater grunts and stuff. Eventually, she'd pick on the language and come to speak Leaf Eater well.
Also, I would need to have Sharpteeth sing in at least some parts of my fanfic.
I admit, sometimes one must ignore cannon. For instance, I'd have Ptero, a Pteranodon/ Sharp Beak hybrid able to speak in English, as is his Sharp Beak mother, yet he is shocked that Leaf Eaters can talk.
Also, the caption thing limits multi-character dialogue so if I had a whole herd of Sharpteeth, some wondering if Chomper is nuts for liking Leaf Eaters as friends while others are ok with it, it might be better to have some English version of them.