Slavica played for the Croatian under-16 junior national team at the 2013 FIBA Europe Under-16 Championship. He helped the Croatian under-18 junior national team win the bronze medal at the 2014 FIBA Europe Under-18 Championship. He also captured a silver medal with Croatia’s under-19 junior national team at the 2015 FIBA Under-19 World Championship, while also receiving an All-World Championship Under-19 Honorable Mention status by the basketball website "eurobasket.com".