Compared to the population of the Netherlands (even if we include Flanders) the population of Wallonia is too big to Dutchify. The Walloons would simply play a too important role in the Netherlands to be so oppressed that they would abandon their own language. Not unlike that the Walloons weren't able to switch most of the Flemish to speak French. Or just like the Germans weren't able to get the Poles to speak German.
Wallonia would be less than 1/5 of the entire population of Benelux power as far as I can tell(today it is just above 10%), possibly even less so. Germans did manage to get many Poles to Germanize, just not as fast or exactly where they wanted(plus they tried more to resettle German than to assimilate catholic Poles). And the Walloons did get many Flemish people to speak French, with the biggest example of Bruxelles and its surroundings, over the course of a little bit more than a century.
Maybe it wasn't made entirely clear, but I was specifically speaking about the high/late medieval and (early) modern period. I believe you can get Wallonia to speak a Germanic dialect with a POD in the early medieval period with a different Migration Period.
I'd argue the complete opposite, you can't just push thousands more Germanic people in any given area, you need the language to spread organically as well, there is a reason why Moselles Romance declined or why German spread in Switzerland during the late middle Ages.
After that age France was basicly the major power in the region and French became (at least in the early Modern period, but I guess it happened earlier) the prestige language and the linguage Franca of Europe.
Well just avoid that then, quite easy with a POD during the late 9th century, and not that hard later on even late medieval(12th century for example) allows you plenty of time, considering how many large scale linguistic change happened all over Europe during this period, I'd go for a 11th century one if one wants to avoid Francization of Picardy.
Also some language related to yours being a big deal in some upper circles doesn't prevent linguistic assimilation, German spread in Arpitan areas in Switzerland despite the status of French.
It becomes especialy hard since noone realy cared about the local language in those days. There would be no incentive for the Dutch or German state to force their own language on the Walloons. That would only happen in the 19th century and than it would be too
late.
It wasn't solely a 19th century thing, it happened before as well even if not directly connect to nation building, some religious movement could get behind that as well, especially in the early modern era.
I imagine having Flanders, Calais and northern Picardy in a Eastern Frankish state from the start would help reducing assimilation to French and set up a situation where a reverse situation could happen like happened with Moselle Romance or Rhaeto-Romance and partially Arpitan.