The note that Mater writes, has it always been in a different language?

Yes.
I have seen Cars 2 in German, and the Letter was written in German,
so i'm sure about it... :)

What are Du talking about? Mater has always spoken in a different language! He doesn't speak normally because of his southern accent, he uses incorrect syntax and I think he's autistic!

Mater's language is different cuz he's Autistic and he has a learning disability. HE'S NOT DIM-WITTED! HE'S A GENIUS! He just has a hard time socializing with others.