Some mnemonics are missing
So it seems like Text Editor has a pretty good coverage of mnemonic support, with usually forgotten things like mnemonic on a label to activate its check box (i.e. in the search bar's settings).
Nevertheless, there's some missing. I've gone through all places in Text Editor, and the only parts that are missing mnemonics are the "Open" button in the headerbar, and all the menu items in the "Configuration" menu button (the one between the main menu and the "Save" button).