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Regardless of if this map is accurate or not, it shows the Rhineland as romance, and I once read that the Rhineland only became germanised in the 15/16th century.
So how can we get the same to happen to Wallonia? I imagine the region has to be somehow held by the HRE, the Netherlands or Austria without the French coming in with their hard-on for annexing the region... Idk how to achieve this, but see how that changes things down the line.
Alternatively, only the majority of Wallonia becomes germanised whilst the rest remain romance speaking.