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The Germanic tribes of Roman times were the Franks, who took over the old Celtic territory of Roman Gaul etc, and their distant relatives we know as the Germans crowded in from behind them.

well, not completely. The Germanics were divided into North, West and East Germanics

Around the time of Christ, the North Germanics like the Swedes, Danes, Norwegians lived in central or northern Sweden and in Norway.

What is now Denmark was the land of the Angles, Jutes, Heruli, Rutians etc., while northern Netherlands was Frisian and the parts of present day Poland around the Vistula river was Goth (the Goths or Geats having originated from the Gothen area of south-east Sweden)

the Goths moved east to what is now Ukraine and towards what is now Romania, taking over depopulated lands (after Trajans genocide of the Thracian/Dacian peoples), while the Heruli moved to the mouth of the river Don and fought the Sarmatians and Alans. The Sarmatians also moved west to what is now Austria

And the Angles and Jutes seem to have packed up and left to England (along with the Heruli who seem to have moved back to Denmark around the 300s), while the Goths separated into Eastern and Western Goths (Visigoths and Ostrogoths) and moved into the Roman Empire

at the same time the Langobards from northern Denmark moved into Italy to become the Lombards and the Burgundians from the Bergen Island between Sweden and Denmark moved to eastern France

The Franks were a coaliation of various tribes, like the Marcommani were or the Alamanni were

The Franks were the originators of both France and German -- Western Francia became France and Eastern Frankia became the Holy Roman Empire of Germania. The middle, Lotharingia was the origin of the low land countries