When Thor introduced Groot to Captain America, the reply from Groot was "I am Groot" .. what is he saying in his language here?
We don't know...
...but it's likely he's saying exactly what he actually means.
Oh, by the way, this is a friend of mine. A tree.
I am Groot!
I am Steve Rogers.
The joke here is that everything Groot ever says in his language comes out as "I am Groot"...it's just that in this specific instance the sounds match what he would say in these circumstances.