It was first reported by Asia Times few days ago. As I previously suggested, France opted to send foreign troops to Ukraine in order to avoid triggering NATO's Article 5, which was a wise move by Macron. They will get French citizenship after serving in the Foreign Legion for three years. There are many migrants in both Russia and France, who are desperate enough to sign a military contract to gain citizenship.
France has sent its first troops officially to Ukraine. They have been deployed in support of the Ukrainian 54th Independent Mechanized Brigade in
asiatimes.com