Is this correct? I thought it was only competitive FULL internationals that generally tied a player to that country. I would be interested to see a source, if you have one.
Any Player who ... [assumes] a new nationality and who has not played international football [in a match ... in an official competition of any category or any type of football for one Association] shall be eligible to play for the new representative team only if he fulfills one of the following conditions:
(a) He was born on the territory of the relevant Association;
(b) His biological mother or biological father was born on the territory of the relevant Association;
(c) His grandmother or grandfather was born on the territory of the relevant Association;
(d) He has lived continuously for at least five years after reaching the age of 18 on the territory of the relevant Association.
— FIFA.com
In January 2004, a new ruling came into effect that permitted a player to represent one country at youth international level and another at senior international level, provided that the player applied before their 21st birthday
Not sure now. The second part rules out Knockaert, anyway.