In The Lord of the Rings: The Two Towers, at Helm's Deep when Aragorn was commanding the Elves in Elvish language during the battle, he yell's

"Ladders"

A moment after that we hear Gimli saying -

"Good"

because he was very eager to fight them and finally the wait was over.

But how did he know what Aragorn said? How did he know Elvish?