Grugach and Grung don't speak common.

What languages do Grung speak?

Grungs spoke their own frog-like language, called "grung," and most did not learn additional languages. Grung made use of chirrs, croaks (transcribed as "roook"), and ribbits or chirps (e.g., "erp").

Do all DND characters speak common?

All Player Characters can speak Common by default, as it's less a language and more of a handwave to say there is no language barrier. That being said, other languages can be gained from your character's race, background, class, and feats. ... Exotic languages are spoken by rarer creatures that are normally not playable.