The industralisation of Wallonia will continue, no matter what, this is an economic factor not a matter of language.I am skeptical about this. Unless we are going to assume a complete marginalization of Francophones—something that I think we may not be able to have, at least if we are to keep the industrialization of Wallonia on track—you would still have south of the language frontier a situation where the local population was almost wholly Romance-speaking and where French had been the established prestige language since the immemorial. There is too much momentum to easily overcome, and even larger Dutch-speaking minorities moving to predominantly Francophone communities would still be minorities.
That would be a lot of Dutch-speakers, IIRC well over a million, with more than a million migrants coming. Is that likely? And why would they not assimilate, as individuals, in much the same way that (say) native speakers of Swiss German did into French Switzerland?
You could make things worse, but that would need a decidedly anti-Francophone shift. Is that necessarily likely, especially if the Dutch monarchical state does.not try to overthrow the established and largely Francophone elites of the south?
What mechanism do you suggest for this?
Francophones will not be marginalized, this is not possible since it is the language of the upper class, also in the North in the first halve of the 19th century and beyond. However it will get out of fashion during this century and in the South possible faster as in OTL.
During the 19th century there was a very large migration from the rural area's to the industrializing cities. In this united Kingdom where in the South the Dutch language is NOT marginalized as in OTL the origin of the migrants will not only be the Flemish provinces but also the rural provinces of the North like North Brabant, Gelderland, Zeeland and Overijsel.
In the South the industry was not only concentrated in cities like Mons, Liege and Charleroi but also in small existing hamlets or new villages close to the coal pits. This will turn small mining communities faster in Dutch speaking towns, if the majority of the inhabitants will be Dutch speakers, combined with Dutch speaking Clergy. And yes this migration will be very large.
The early industrialization of Belgium in OTL was impressive but it will have a multiplying factor in an United Netherlands. Nearly every thing would have a multiplying factor in a United Netherlands compared to individual nations of Belgium and Netherland.
Any how there will be no marginalization of Wallon in a United Netherlands since there are no suppressing language rules or laws as in OTL Belgium with Dutch.
Brussel will remain a Dutch speaking city, no marginalization of Dutch. Brussel will grow over time as the main capital city.
Several factor will contribute to this; Willem II preferred Brussel over the Hague, and a growing and more complexing State apparatus.
A Dutch speaking Brussel in combination with a migration from North to South where the migrants can keep there language will gradually shift the language border. Even this will not be a clear line , over time cities as Mons and Charleroi could become cities were Dutch will be more dominant, more or less as happend in OTL with Brussel. A city as Liege and Namur on the other hand would most likely remain Waloon.
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