I am not good at english but I am going to try to explain it.
I don't know these are informal or formal. looks like informal.
말 해도돼 -
it depends on the context.
to see there's no question mark, I assume this as a plain text.
The one who was listening could feel uneasy to say something.
So the speaker is trying to make someone say without worrying about the facts that someone has
It's like "you are allowed to say that or yo can say it"
and It is like a contraction of "말 해도 된다"
저녁은요? - Did you have dinner?
It's very commonly used in daily conversation.
but it might look a little weird to non-native speaker.
You can understand this like "저녁은 (드셨어)요?"
But you should not use '요' to one who younger than you, just "저녁은?" is enough.