I don't usually write a lot of emails on my iPhone and also rarely use French to do that.
The other day I had to write a email on the iPhone, in French.
Here's how easy it's to do that, just a few steps:
1. Enable the French keyboard:
Settings > Keyboard > International Keyboards > Turn On the Switch next to the desired language (French)
2. Create a new email, hold the Globe key (the one in the left of the Space key) and select the new language.
It's that easy and the cool part is that it comes with spell check for that specific language.