Thor is one of the only ones to understand Groot, and it may well be the only one to have understood its real name.
If you are asked what is your favorite character of Guardians of the Galaxy, we bet you will respond to 89% Groot ! Adorable, fierce, fearsome, but cute as anything, there has been the faithful companion of Rocket to go through several phases between the two parts of the adventures of the band of Peter Quill and Avengers Infinity War ! But do we know really ? Not sure. After all, Rocket, Gamora and Peter have learned to understand over the years we spent together, but they don’t seem to really speak his native language. So, Star Lord may have allowed Thanos to make her snap, but it also allowed us to know the true name of Groot in collecting Thor in his ship. The God of Thunder himself says, he has learned the language of Groot on Asgard !
However, when he presents it to Captain America, he introduces it as “Tree” (tree) and not Groot. Would it mean that he alone has understood her true name and that others have just assumed that it was called Groot, in his own way to repeat it ? It is very possible ! It is hard to see Thor, rather respectful, in general, call Groot “Tree” for no reason, as he called him “friend”. It is also possible that Groot is his first name and “Tree” is his family name, given that Thor regularly uses the names of family, as noted by this user Reddit. All the world would that be so wrong since the beginning ! We’ll have to go all on Asgard take language courses developed. With Thor of preference. On the other hand, he apparently has not learned to decipher the women, otherwise, he would have realized that Gamora would be able to beat him in combat !