Now the new rule is that plugins don't include any other language than English.
when you install the certificate plugin, if your website has the French language installed, the French strings for the plugin are downloaded from AMOS http://lang.moodle.org/ and it works . Same way if you update the installed lang pack, languages strings for all plugins are also updated.
Of course the condition for this mecanism to wok is that people (and developers) contribute to AMOS by adding translations. It's very easy, I have just done it for forumng and French language a few weeks ago. You don't need to know anything on programming, you just select he language, the plugin and the version and missing strings are displayed, you translate them online, the maintainer for this language accept your work and that's all.
Now back to your customization problem, first if you think the translation from AMOS is not correct you should login on http://lang.moodle.org/ and submit a correction.
But if you only want to change it on your website, go to Site Administration -> Language -> Language customization open the French language pack, click on "Continue" button, find the certificate component in the list and select itt, click on "Show strings"
Find the string you want to change, enter the desired text in the last column and save.
This the best way to customize a string in Moodle because your changes will no be overwritten when you upgrade your languages packs.
See http://docs.moodle.org/25/en/Language_customization (or http://docs.moodle.org/2x/fr/Personnalisation_de_la_langue ) for details.