It's in wiki!!
Kalidou Koulibaly was born and raised in France by Senegalese parents, and was therefore eligible for both nations. He played for the U-20 French side starting in 2011. However, in early September 2015, Koulibaly changed federations and joined the
Senegal national football team, despite the interest to call him for Les Bleus from
Didier Deschamps.
[12] He made his debut on 5 September 2015 in an AFCON qualifier against
Namibia in a 2–0 victory.
Yes, Niasse plays for Senegal, but so does Sadio Mane, Demba Ba (or did) etc.
Lee Dixon played for England, but so did Gazza, Lineker!