There are two ways your request can be read. One that you are trying to find a French language package to install. Another one that you are trying to create a French language package yourself. I will reply to both questions.
If you are looking for an installable language package
for your language be advised that we have discontinued them
more than a year ago.
If you are actually trying to create your own translation, the language files are located in Joomla's language/en-GB and administrator/language/en-GB directories, just like every other Joomla extension.