In the current GUI an IMAP "INBOX" will be displayed as "Inbox" in the folder list in the side bar.

However it is displayed as "INBOX" in:
- the (bold) label of the side bar;
- the folder properties dialog;
- the window title (as displayed by the windowmanager).

I suggest to replace these three occurrences of INBOX in the GUI code with Inbox too. INBOX is a written in all caps because that is the way it is used in the IMAP protocol. There's little reason to use it in the GUI. Inbox is properly capitalized and can be localized too.

I'll attach a patch shortly.

(Note that even without my patch the INBOX is also displayed as "Inbox" in the IMAP Folder Subscriptions dialog. I assume this is done by the current "INBOX" renaming code, which I assume is found in em_folder_tree_get_selected_folder_info() in mail/em-folder-tree.c. One might argue that in that dialog INBOX is actually more appropriate, as subscribing to IMAP folders seems to me to be something where using more exact terminology is probably better.)