I'm not in a hurry, since I already have it running this way on my machine. I just submitted the PR because nested menus aren't great for features users might actually want to use. They're fine for things like filetype and encoding that are supposed to be autodetected.
For the mnemonics, is preferences 'S' for settings? Is it so frequently accessed that it needs a mnemonic? I'm looking at other programs, and they each seem to use a different letter.