Here via...I think mirabellwr's LJ. This is exactly what I was going to post. I often find that when I'm speaking with Germans, French, Italians and/or Spaniards, that we switch back and forth, more or less randomly. For business speech, of course, many technical terms only exist in one language or another, but for casual speech, often one person only knows English and their own tongue or that sort of thing and it does lead to rather patchwork conversations.