Go back far enough, and the area of Belgium was part of the Holy Roman Empire, which starting around the 15th century was called Holy Roman Empire of German Nation. Check out this map showing the 1618 borders. The people in the area of Belgium probably didn't have much of an identity separate from the HRE (and their village) yet and might still have considered themselves "deutsch" (which is where the Dutch got their English name from) depending on the context.

IMO, the question of "when was Germany founded" just isn't as clear-cut as "when was Belgium founded". Many modern states would consider the year they first became an independent polity as their foundational year (or at least as a foundational year), rather than the year the current state construct was founded. e.g. consider how the Chinese keep going on about how China was one state for thousands of years. In that sense, "Germany" was founded when the Carolingian empire was split in three.