Add French to fully-translated languages (#6305)

Author: tdd

src/components/Seo.tsx

@@ -21,6 +21,7 @@ const deployedTranslations = [
   'en',
   'zh-hans',
   'es',
+  'fr',
   // We'll add more languages when they have enough content.
   // Please DO NOT edit this list without a discussion in the reactjs/react.dev repo.
   // It must be the same between all translations.