That's a really interesting question, and I can speak best about the Franks. The thing we're most certain about is that our Frankish sources are biased. Charlemagne's court justified their brutal conquest of the Saxons by calling them uncivilized. Charlemagne's campaigns might have been called genocides if they had received the level of media coverage possible today. Archaeologists now know that the Saxons had just opened up silver and garnet mines, and one of Charlemagne's first interests in the Saxon's territory was trying to dig a channel between the Rhine and the Danube. Clearly he respected the Saxon peoples for their economic potential, and the brutality with which he oppressed them speaks to his respect for their political potential. Neither of these come across in the sources, but actions speak louder than words.

We also have written sources by Frankish churchmen who were sent (or sent themselves) as missionaries to Baltic areas. These were men with an agenda. Most are from the archdiocese of Hamburg-Bremen, which was in an increasingly precarious position after Charlemagne's death. His successors focused on civil war rather than maintaining their borders, and Hamburg-Bremen became an isolated outpost of Frankish society. The Life of Ansgar (mid-800s?) is our best source, and Ansgar clearly seems reluctant to stake his life on the line to proselytize among the Baltic tribes. But there's a few fishy things going on in this account.

First, the paganism of these tribes sounds suspiciously like the pagans of rural Mediterranean areas in earlier saints' lives. That is to say, this is an established literary trope, not accurate reporting. Second, the barbarity of the Baltic tribes throws Ansgar's own missionary heroics into stark relief, which furthers the author's purpose of making Ansgar look like a saint, and should likewise not be trusted. Third, the dangers of the barbarous peoples of the north are emphasized to justify financing more missions to civilize them. This text isn't about offering an objective account of the Baltic tribes; it's about court politics. Ansgar is a saint because he acts like other saints, and we should send money to support his shrine and his successors.

That doesn't mean the Franks were ignorant of the Baltic tribes; nor were Baltic tribes ignorant of the Franks. The best example is Godfred (whose name is spelled differently pretty much every time you find it). The Franks had a trading exclave on the Baltic, kind of like how Britain used to administer Hong Kong. Godfred realized how lucrative it was to control and tax trade, so he pillaged the town and forced all their merchants to move north into his kingdom in Denmark.

Charlemagne's response tells us two things. First, he launched a massive campaign against Godfred in the year 810, which tells us that he considered Godfred as a legitimate threat, perhaps even a potential rival. Second, Charlemagne sent on campaign a war elephant that he had received as a gift from the caliph in Baghdad.

By appealing to his connections with the caliphate, Charlemagne hoped to show that he had credentials that Godfred couldn't compete with (He was wrong: more Arabic trade moved north to the Baltic than west to Francia). And Charlemagne showed his sensitivity to the importance of exotic goods as a source of legitimacy among political leaders in the Baltic. He knew and respected how Baltic politics worked, and he wanted to have his own say.

Looking at the British islands, the Franks were eager to increase their influence. Kent, in southern England, shows evidence of intensive exchange beginning by at least the 600s, and it suggests that there were a lot of aristocrats - and maybe even more common people - marrying back and forth across the Channel. They also took every opportunity to send Frankish churchmen to bring the British some orthodox Christianity ... and Frankish influence as well. You can almost sense their desperation when the Northumbrian kings (who controlled part of today's Scotland) began talking directly to the pope to sidestep Frankish ecclesiastical appointments.

Byzantine Rome isn't really my area, but in the 900s they started employing Scandinavian mercenaries in the renowned Varangian Guard. And Norse-speaking merchants got some pretty lucrative trade agreements for using the docks in Constantinople. Call it a hunch, but I don't think you'd trust an "uncivilized" people with your empire's security and trade.

In short, I think "uncivilized" is more of how we'd like to think about it, but that basically derives from the fact that these people didn't leave written sources. The people that did write - Franks and Byzantines - seem to have often regarded these people as equals, or at least to have treated them as such, which is probably more important.

Compared to the caliphate, the Franks and Byzantines may have seemed like small and insignificant kingdoms. (Arabic sources don't even both mentioning that they sent Charlemagne an elephant.) And judging by the flow of silver into the Baltic, the caliphs were more interested in Scandinavia than in their neighbors on the north coast of the Mediterranean.

Sorry this has gone on so long, but it really is a very interesting question. And sorry I've bounced around over a few centuries' worth of evidence, but the early medieval world was an exciting and dynamic place! Cheers!