Vim is known to be a powerful text editor, and it's true. With this simple command, you can enable spell checking (replace "fr" with your language code):
:setlocal spell spelllang=fr
If this locale is not installed for VIM, it will offer you to download and install it.
Once the spelling is activated, the misspelled words are displayed in red. Now you can use the spelling commands:
]s -> move to the next (probably) misspelled word z= -> suggest corrections zg -> add this word to the dictionary
Interrestingly, if a couple of words are valid only together, you may select them in visual mode and then hit zg.
Happy writing!