The answer is: In the user's /Library/Mail/Deleted Messages.mbox/
Messages folder.
The string "Deleted Messages" is stored in the preference file
"com.apple.mail.plist" in the key MailAccounts/0/TrashMailBoxName.
Maybe. First, that's actually a preference (see Preferences >
Accounts > Mailbox Behaviors > Move deleted messages to the Trash
mailbox); second, messages for an IMAP account aren't necessarily
stored on disk at all.
I suspect that the value of this key depends on the language
preference set in the International preference pane when the user
first used Mail.
I'd be surprised if that were the case -- it would mean that your
trash wouldn't work if you switched languages. Try changing primary
languages and see what happens.
