(Apologies for the late reply!) We normally went with guillemets (« and ») and a different font to represent English dialogue in this translation, but for Kimura's bit in particular, the fact that it was the original source of "mai waifu" as Internet slang made writing out the romaji transliteration the more obvious solution to us. It also helps to convey the original strangeness of Kimura replying in English to a question in Japanese; the anime's dub went with "mein Wife" to convey it in a similar manner.