After my wife read this post, she started telling me about what might amount to an interesting trend. After noting that she herself prefers to argue in English rather than in Japanese, she said she heard the same thing from her Japanese-speaking American professor about his English-speaking Japanese wife. He told her that typically he'd be using Japanese while she'd be using English when they argue.
She cited two reasons for why she prefers English. The first, which I don't buy so much, is that English has more appropriate curse words to throw into the mix. I would agree that English has a leg up on Japanese in the curse word department, and we certainly use them a lot more, but Japanese attains the exact same effect through intonation and certain verbal forms rather than adding colorful vocabulary to the sentence. For instance, the standard, informal way of saying, "What are you looking at?" would be "Nani miteru?", "Nani miteru no?", with the no making it a bit softer, or "Nani miten no?", with the swallowing of the ru to an n making it sound more informal. You start to sound unhappy when you say, "Nani miterun da?" or, with a bit more oomph, "Nani miterun da yo?". When you're even more unhappy, so unhappy that you can't even say the whole thing, then it'd become "Nani miten da?" or, with oomph, "Nani miten da yo?" Depending on how it was said, that last one might be translated as "What the hell are you looking at?" To upgrade that to the equivalent of a stronger four-letter word in English, you would just make the intonation more angry, forceful, and emphatic.
The second reason, which makes a more sense to me, is that she said that when she switches languages, she switches cultures as well. I can relate to this better, as this is something I do as well. When visiting a friend in France with whom I had studied in Japan, she joked that she could tell what language I was speaking - English, French, or Japanese - without even hearing what I was saying due to the change in my mannerisms. As for arguing, Japanese culture is quite a bit less confrontational than our barbaric Western culture, and argumentative females remain a relatively rare species in Japan, so the cultural switch leaves her at something of an arguing disadvantage.
So I'm curious... has anyone else encountered anything like this in a multilingual relationship?