Well growing up bilingual, (French and English) in Quebec, all my friends speak a mixture of both languages. It's nothing specific, it's more like if you can't remember a word in one language you say it in the other. My southern US born DH, who happens to have a BA in French and German, has a really hard time understanding my friend Shirley and I, or even my DS and I ,when we really get going speaking a mixture of french, english and old style joual.
It amazes me how languages change. Very similar to the change in tonality and spelling is the story of the Louisianna Cajuns, decendants of the Acadians that were deported from Canada by the conquering Brittish in 1755 and 1758. Both the French Canadian Joual of Quebec, New Brusnwick and Northern Ontario and the Cajun French of Louisiana are virtually impossible to understand by a person living in France or any other country. And even more interesting, because both joual and cajun continued to evolve independantly of each other, they can't understand much of what the other says either. Of course, since neither joual nor cajun are what children learn to read and write at school I know that my journaling is in "real french baring a few coloquialisms and I'll bet that there are few scrapbooks that hold cajun french journalling.