Reported by Vatadoshu at Wikipédia:Flow/Bugs.
The two messages <thanks-thanked> and <thanks-button-thanked> are only given the name of the current user, but the French translation (“remercié” or “remerciée”) depends on the gender of the thanked user.
This is done from javascript, so likely the api calls need to return information about the gender of the user, or make a second api call (https://www.mediawiki.org/wiki/API:Users). Can probably do the two api calls in parallel