The movies do Groot dirty (and the rest of the team, too, for that matter). He's not a one-word simpleton - in his species, their equivalent to vocal chords harden as they age, restricting their vocabulary. In short, he doesn't say one word because he only knows one word, he can physically only say one word.
I mean, this comic does. And the movies at large turned every character into simpletons that at any given moment are saying or doing the dumbest possible thing they could.
Also, Rocket pretending to understand is very in character, but I haven't seen any guardian movies past 2, so I don't know about Thor.