The East Germanic language was extinct long ago, and there was once a time I hoped they speak Dutch/Flemish.
I know in-game they speak a different dialect of French, because when I click on a villager, their pronunciation of “bonjour”, “pret” sounds rather different.
The Burgundian units speak their namesake (Burgundian: Bregognon, French: Bourguignon). It is an Oïl language spoken in Burgundy and particularly in the Morvan area of the region.
Why would they speak a completely irrelevant language?
It’s not completely irrelevant. The Burgundians also represent the Low Countries, like Belgium and, yes, the Netherlands.
Then the civilization name should be dutch not burgundians.
Civilizations can represent multiple things. Burgundy controlled both Belgium and the Netherlands at one point in time.
Netherlands should speak Frician not flemish.
English controlled scotland and ireland does that mean they all should speak english?
The Frisians should speak Frisian. The Frisians were not the Dutch. They were one people group living in the Netherlands, not the whole country.