Linguistically, the Scandanavian language (and some linguists insist that Norwegian, Swedish and Danish are all just dialects of each other) are part of the Germanic language family -- the Germanics only really come into history around 100 BC with some ROman/Greek's writing about them as barbaric peoples beyond the Celts
Even as late as the 300s AD, the Goths, Geats, Vandals, Alammani, etc. speak a mutually intelligible language, so the split was probably relatively recent with it solidifying only perhaps about 900 AD. Perhaps around 200 BC the Germanics were all peoples around the southern coasts of Norway, Sweden and the northern coasts of Germany, the Netherlands and Denmark and probably the north-western coasts of Poland (Wolin etc)
The Celts seem to have exploded around 300 BC from the Hallstadt culture, invading Rome, invading Anatolia and probably Iberia, maybe ireland at this time. What prompted this? And what prompted the later Germanic movement south?
Someone I know who (for some reason) learned Norwegian in recent years sort of denied that Danish and Norwegian (which are more closely related than is Swedish) are all that similar. :’) Somewhere I read that Norwegian speakers are more numerous in the US than they are in Norway, and of course, it’s not like spotting a toupee’ — I worked with a guy four or five years, and just before he moved on to greener pastures he noted that his parents had come from Norway, and that he was fluent in Norwegian.
Some of the Germans of whom the Romans spoke were the Franks, oddly enough a Germanic people, but their conquest of the former Gaul led to a deviance in their spoken language due to Celtic tongues and Latin, and who knows what else. They wound up allied with the Romans more often than not, fighting against incursions from the east.
The steppe being an east-west great plains-like prehistoric superhighway was handy when the natural climate cycled downward, resulting in an emptying out of Central Asia into Europe (for that matter, into Iran, India, and China) after some centuries of really good weather had brought on a boom in population. The Indo-Europeans wound up in Europe, arriving in pulses.
Jutland in Denmark was the old home of the Jutes, invaders/immigrants who settled in post-Roman Britain and founded the Kdm of Kent (the old heptarchy was Kent, Sussex, Wessex, Essex, East Anglia, Mercia, and Northumbria; other shire-sized kingdoms gradually vanished during the so-called Dark Ages, but often their capitals have survived nicely, along with folklore about them, sometimes genealogies); the Saxons have served as the villains of the King Arthur stories, but it was Wessex which led the resistance to the later Danish invasion, yet another Germanic people. The Angles wound up getting the whole of England named after them, not bad.
There’s often speculation about the Berbers of N Africa, how they came to be so fair skinned, blue-eyed, blonde, etc — that’s where the Vandals wound up, and while their language probably vanished (other than transliterated loanwords), their genes have done pretty well for themselves. The Vandals’ earliest appearance was in Scandinavia, then down into modern Poland, up against and along the Danube, then they were pushed by the Huns across the Rhine into Gaul, then down into modern Spain, finally into N Africa where they took over everything, and expanded into the major islands, and even sacked Rome (Rome was probably used to it by then). They appear to have been pretty civilized for barbarians, and skilled with goldwork.