In Avengers: Infinity War Cull Obsidian responds verbally to his mates but it seems hard to comprehend. At least for me I have no clue as to what he says. What were his responses or what language does he speaks?
Cull is speaking in his own native tongue. We don't know what planet that is, but at least we know that he's using words.
Cull Obsidian added something vicious sounding in his native tongue, which sounded more like a series of grunts and growls. As Tony grew more and more impatient. Maw continued. "Be thankful that your meaningless lives are now con—"
Ebony Maw sighed in exasperation. Sensing Cull's desire for a fight, he waved his thin hand toward the quartet facing them. "He exhausts me." Cull Obsidian answered Maw in his own unintelligible language, bringing his weapon up into battle stance. "Bring me the Stone." At Ebony Maw's words. Cull Obsidian grunted in agreement, and slammed his massive hammer into the New York pavement, cracking it as if it were made of the thinnest sheet of ice.
Unfortunately, we don't know. I initially thought he was only grunting but after paying attention and re-watching the introduction scene with him and Ebony Maw on Earth he certainly is making words not just noises. I've checked all the common places and no information is given. The closest we get is the subtitles on Disney+ which simply say:
(SPEAKS ALIEN LANGUAGE)
The Road to Marvel's Avengers: Endgame - The Art of the Marvel Cinematic Universe very briefly says:
As brutish as he is big, Cull Obsidian is a creature of few words, preferring his fists to do the talking.
Again this implies that he does actually speak but once again no mention is made of what language.
In fact of the loyal members of the Black Order we don't get any back story to them just that like Nebula and Gamora they were taken as children from their home planets. Unlike those two though we don't know what planets they are. The Black Order's backstories just aren't filled out in the MCU as far as I can tell.