Can everyone understand Stewie Griffin

Answer 1

Seth has said that everyone is able to fully understand Stewie, but everyone treats him as the one year old child he is. For instance, Stewie recognizing himself as a balloon in the Macy's Thanksgiving parade and Lois simply humors him saying to Peter, he's a baby and he sees another baby on TV.

Obviously Brian (& the few scenes with Jasper) and other babies fully understand and communicate with him regardless of the scene.

He's not taken seriously unless the scene requires the other characters to do so.

In several episodes, including the one where the family goes into the safe room, away from the robbers and the one where Brian, Peter, and Stewie are in the bathroom, episode unknown, both Lois and Peter talk to Stewie.

Lois (to Peter): I, I have to agree with Stewie, Peter.

Peter (to Stewie): Oh, I don't think so Stewie.

In the above example, it may have been a gesture or physical/facial reaction that made Peter or Lois respond in this way.

Chris asks Stewie directly about the Ewok joke he told.

In the episode when Cleavland and Loretta break up and Stewie and Brian go to talk to her. BEFORE they knock on there door they have a private conversation as if they are off the set. In this conversation the mystery is revealed;

Stewie: "is lorretta one of the characters that can understand me?"

Brian: "you know i think she's close enough to the main characters to understand the jist of what your saying.."

guy 1 (off camera): "Were rolling!!"

Stewie: "oh right right right right right."

*rings door bell*

Stewie: says something about how glens crushed blah blah

Loretta(TO STEWIE): "you know stewie i only really hear the jist of what your saying" - The end.
All of the family hear Stewie to some degree; most do not fully comprehend what he actually says and treat him as if he was burbling in babytalk.