I write in English and French. How do I add french?
Bonne question. But you didn’t tell us your Operating System or version of LO. Please see the comment above by @gabix.
You need at least two things, but please note that only one of the things listed below involves LibreOffice :
to internationalize your keyboard or to use a character map to give you access to accented characters. Both Windows and Ubuntu Linux come with a character map. Using a character map is extra work, but it works. Instead, I save the most common accented characters in my clip-board manager.
a French spell-checker. (I use Ubuntu and have not found a French spell-checker. Maybe someone else can help with this.)
a French dictionary installed on your computer. (I ask my wife or look it up in our petit Robert.)
check out the LibreOffice extensions for French-language tools that might help you.
Additional info to @ve3oat’s answer:
Under Fedora Linux (and many other flavors of Linux), the spelling checker engine is called Hunspell. It is a separate product from LO. It is likely it is already installed because it is referenced by many other applications. Otherwise look for a package named hunspell.
Install the French rules for Hunspell. Under Fedora, the package is named hunspell-fr. Adapt to your distribution.
Dictionary are application-specific. For LO, they are provided by package libreoffice-langpack-fr. Adapt to your distribution.
If you’re under Windows, relaunch the installer to add the language components you need. Note that this does not solve the issue of the spelling checker issue.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer which is reserved for solutions) or comment the relevant answer.