Change regional mailbox after added emails

Hello all,
I have one question.

Can we change language for my all emails automatically because they are using the protocol IMAP? I don't want lose messages. First time I added emails using language polish but it was too late and I changed polish to german and no result. I'm using Thunderbird, configured emails, received messages but these folders are using polish... but I want see folders with deutsh.

I'm sorry for my English.

Robert Tomiczek