This is where it is important to remember that this was a 'dynastic' union. The Netherlands and Castile had the same monarch, but not the same government. For that matter, Castile and Aragon had the same monarch but not the same government/administrators. Each of these possessions was ruled through existing institutions that were not centralized. The same was true during the period between 1580-1640 when Portugal was ruled by Philip II and III.

It is true that because of the closeness of these kingdoms during the dynastic union you could get officials and appointees from one kingdom operating in the other kingdoms, but ultimately the monarch ruled each separately and by the legal frameworks that they each independently had.

So there was no reason or plan to have Spanish become the language of the Netherlands. At the start under Charles, his Castilian was not particularly good when he arrived to take up the monarchy. Obviously Phillip II and III were raised with Castilian being a much more important language but they would never have considered making it the official language of their holdings in the Low Countries. For that matter, they never forced Castilian on their Indigenous subjects in the Americas. Linguistic plurality was the norm and the procedures and practice of government accommodated that variety.