English - although keyborad is predicts bilingual - and there doesn't seem to be a way of turning it off.
However, it does highlight second language as none.
Sometimes it is just english. I gets upset when I email my sister in france, and then stays french!
Ah - if you email her in French, perhaps it's smart enough to remember the language from the previous one, do you have to tell it to revert to a default language? You didn't by any chance buy it in France?