So, in countries (like France) where Monday is regarded as being the first day, how is that gospel passage translated?
Business calendars start on Mondays. Secular calendars star on Sunday. The important thing is what day does your pay period start.
I checked on www.BibleGateway.com, which has four French editions.
La Bible du Semeur uses “Le dimanche matin.”
Louis Segond and Nouvelle Edition de Genève use “Le premier jour de la semaine”.
Segond 21 uses “Le dimanche”.