Case C-801/18: Order of the Court (Sixth Chamber) of 5 September 2019 (request for a preliminary ruling from the Conseil supérieur de la Sécurité sociale — Luxembourg) — EU v Caisse pour l’avenir des enfants (Reference for a preliminary ruling — Article 99 of the Rules of Procedure of the Court of Justice — Freedom of movement for workers — Equal treatment — Article 45 TFEU — Regulation (EC) No 883/2004 — Article 4 — Social security convention concluded between the Member State of employment and a non-member country — Family allowances — Application to a frontier worker who is neither a national nor a resident of one of the Contracting States to the convention)