"We zeggen dag, voordat we naar België gaan."
Translation:We are saying goodbye before we go to Belgium.
In a subordinate clause (i.e. NOT the main clause) introduced by a subordinating conjunction (here: voordat), the verb goes to the end. This rule doesn't apply if it is a coordinative conjunction (such as 'en' or 'maar'). There are explanations on that in the 'tips and notes' (available from a computer).
No, you can't omit the verb in most cases. The translation of 'I want to go to X' as 'ik wil naar X' is not really grammatically correct, but it's being used increasingly by native speakers, so I suppose that's why Duolingo accepts it. It would be an exception at any rate, yes