I do not think that there is such a spell checker. It's probably very hard to write one, as good approximate suggestions require a somewhat deep understanding of the language in question, and natural language processing is quite a complex topic.
However, there is a somewhat simpler approach to writing French (or generally a foreign language) on a US keyboard: Just switch to an appropriate input method with C-\
.
Emacs comes with a large selection of alternative input methods, for a wide range of languages and scripts, including Kyrillic, Korean, Chinese, etc. A good one for writing French text is probably latin-1-prefix
.
This input method let's you type accented characters by typing the accent first, and then the character. For instance, typing '
first and then a
afterwards inserts á
into the buffer. Likewise, ` and then a
inserts à
. I use this method frequently to write German on a US keyboard.
For more information, consult International Character Sets and Input Methods in the Emacs manual.