السلام عليكم ورحمة الله وبركاته ‎ أحسنَ اللهُ إليكم Before I can officially start to work I will have to say:”I swear allegiance to the King, obedience to the Constitution and to the laws of the Belgian people, and I swear to faithfully carry out the duties entrusted to me.” Is this allowed?

Ustadh AbdulAziz al-Haqqan

It is better to stay away from this and to look for other work, in shā' Allah. An oath that may tie you to what goes against the Shariʿah is something to avoid. If you find no other way, ask about your exact case before you commit. Allah knows best.