Characters like R2-D2 (Star Wars), Chewbacca (Star Wars) and Groot (Guardians of the Galaxy) share the attribute that they speak a language unintelligible to viewers and readers - we have no idea what they say - yet they have no problems communicating with other characters who speak English (or whatever is the primary language of the story).
Which Sci-Fi work introduced this concept?
The character speaking an unintelligible language must be a main character, and must be non-human (e.g. an alien or robot).
We must be able to understand the other characters.
Other characters must be able to converse with this character.
In other words, the dialogue, to the audience, must be one-sided.