Early 15th century Burgundy seems like a pretty culturally diverse place, and I know the term "German" might not have been what people identified as then. What I'm really asking is, did Burgundy, by the 1420's, hold any geographically German areas, in that they would have considered themselves more citizens of the HRE than France? I might be completely off the mark here, and the case might be that everyone in Burgundy considered themselves "Burgundians" and/or vassals of the French Crown. If that's the case then so be it, but if not, what provinces/regions of Burgundy considered themselves, or would we consider today, more German than French? Sorry for the loaded question in advance.
I think you're going to find things much messier than you'd like. Burgundy has been called a "hedged-in feudation", a patchwork of inheritances, dowries, purchases, with various ties to Valois and Hapsburg royal houses. The area around Lille and Dunkirk has been long known as French Flanders, spoke Low German/Dutch, was long divided between England and France. The Low Countries, which mostly spoke Low German/Dutch and Frisian, what we'd think of now as the Netherlands and Flanders, were pretty complicated- Brabant, for example, was for a long while its own country, or Duchy. I would take a look at this nifty chart on the Low Countries, showing the complex shifts in who owned what and when, including when the Hapsburgs and the HRE got it.
In southern Belgium, Walloonia, people spoke French- as they do today. At the court of Phillipe le Bon of Burgundy, French predominated.