Before Julius Caesar fought the Gallic tribes for Rome (58 to 50 BC), most Romans were unaware of “Germans”. Then he called the Barbarians (i.e. tribal non-Romans) east of the river Rhine “Germans” and thereby invented them independent of ethnic or cultural criteria.
These people weren’t Germans speaking German: they were “Proto-Germans”. Around 500 BC, some Indo-European tribes in southern Scandinavia or northern Germany had started to pronounce words differently. For example, their question words started using a (h)w sound like in what? was? or vad? while other languages stuck to a k(w) sound such as in quoi?, qué? or kakiya? This Proto-Germanic language ultimately developed into all Germanic languages ‒ such as German around the 8th century AD.