If your novel is written in a strict PoV style, then your characters from World A will at first not be able to make head nor tail of what people from World B are saying, and
vice versa. They won't even be able to separate the words out properly. (Most people, unless they're consciously trying to be understood by a non-speaker of their language, speak in a continuous stream of syllables, not individual words.)
At this point in the story, your PoV character can tell the reader what they hear: an indecipherable concatenation of noises:
The man said something, but I might as well have been listening to a petrol lawnmower for all the sense he made.
(1st person example.)
Most of the "conversation" will be body language:
She gestured for John to follow her into the cave.
(3rd person example.)
Given that you won't want to bore your readers with too much of this, most of the early encounters between the people of the two worlds will be dialogue-light. This, though, is an opportunity: if a character from World A is in World B, their narration can be more descriptive of what they see (particularly the differences if neither World A nor World B is Earth: you can then describe both World A and World B in a natural, non-info-dumping manner).
As the characters' understanding of each other languages develops, the dialogue will be whatever each of your PoV characters make of what they hear (i.e. the content) rather than the sounds they hear; unless, that is, you want to invent new languages.