I haven't heard Emmylou's version but who the hell can understand everything she sings, even in English? I love the glorious sound of her voice, but she often sounds to me as if she's singing with a mouth full of cotton wool -- so, no wonder you can't understand her singing in Cajun French!

I think the most important thing when singing Cajun songs is to sing with conviction and the right accent/tonality/rhythm. The actual words really don't matter. Dead Horse is quite right. The Cajuns often just take a bunch of stock phrases and string them together to make a song. I have often sung (for Cajun-speaking Creoles) songs that I didn't even know -- I knew the melody and just plugged in "Tu m'as quitter pour t'en aller, oh ye yaie chere, comment tu me fait comme ca, malheureuse, quoi t'a fait avec ton neg" etc. There are some more modern songs (like DL Menard's songs for example) that actually tell a story and have a set text, but nearly all the older songs do not have a set text, with the notable exception of "J'ai Passe Devant Ta Porte" which tends to have pretty much the same lyrics no matter who sings it. But Jolie Blonde does not.
Suzy