In the Guardians of the Galaxy films we can gather by how he says it and by how a character that understands Groot responds as to what was said or asked, but do we have or will we ever get, subtitles for Groot's dialogues?
He(It?) says "I am Groot". There is more to face-to-face communication than words, that is the point, I think. For instance, you can't see or hear me now, so you are missing all the extra context.– JodrellSep 11, 2018 at 11:34
James Gunn confirmed the existence of it in his tweet:
The special Groot version of #GotGVol2 that only Vin Diesel & I have, where every Groot line is printed in English.
As per Gunn, only Vin and Gunn know it fully. And even both confirmed it:
And after Gunn got fired we don't know if he or someone else will ever release the Groot version of the script or not.
6Are we sure that this wasn't a joke? Sep 10, 2018 at 12:13
5I hope nobody gets the yellow and goldenrod drafts mixed up... Sep 10, 2018 at 14:01
2He'll probably sell it for a few millions in a decade. Sep 10, 2018 at 15:34
16@JackAidley: It seems quite reasonable that this would exist so that Vin Diesel knows the appropriate way to say the line and it also makes sense given other characters understand and respond appropriately that there must be "translated" versions of those lines in existence at some point. ie you'd write the dialogue in english first and then later replace all Groot's lines with "I am groot". The thing that surprises me is that the english isn't in every version of the script. Since rocket for example seems to understand everything groot says it makes no sense for Bradley Cooper not to know..– ChrisSep 10, 2018 at 16:09
3@JackAidley added another proof– Ankit Sharma ♦Sep 10, 2018 at 16:18
The majority of the time, we just don't know - we have to infer what he says by the context of the conversation with the other characters.
There's an exception here where Groot speaks to Rocket Racoon at the end of Infinity War Pt 1.
Where Groot says