Sorry, but translating English in Japanese manga into Japanese is just sad. Like someone said here already use of English in manga assumes that most of the reader base will know some basic English to still be able to read it. Can't say that about English readers knowing Japanese, cause if we knew it we wouldn't read the damn translations in the first place.
One comic I read that is English based, but uses Japanese cause most of it happens in Japan IIRC, just uses "<>" to mark sentences that are "translated" Jap to Eng. Something along those lines could be used here with a TL note. Or like the end page - use a different font, cursive, italics, anything that would make it distinctive enough, but still readable easily but without translating it and thus making it unreadable for us readers. Or do translate it into Japanese, but have a smaller English text in or outside the bubble or something?