The following non-Leaf Eaters, Bothteeth, and Gliders can talk:
1.) The Gang DID ask Scuttle about him knowing Leaf Eater and seemed shocked that he could speak it.
2.) Presumably, the Gang just assumed Chomper picked it up from when he was with them.
3 and 4.) Presumably, the Gang was too busy trying not to be eaten by them to bother to ask. (We can leave the mystery of that for a future episode.)
5.) They never asked Mo, at least, not yet.