What do you think is the easiest and hardest language?

Regardless of what, precisely, you mean by "people," we don't really know much about them. Evidence of the first hominids (any species of humans) to live in Europe, Homo georgicus, was found in Georgia and dates back nearly two million years. Several other species appeared later.

H. neanderthalensis, possibly our closest relative, arrived in Europe 150KYA and vanished around 30KYA. Modern humans--our species, H. sapiens--first show up in the European fossil record 40KYA. Since their era overlapped that of the Neanderthals, it's assumed that they simply out-competed them and either drove them away, killed them off, or (today the most widely accepted hypothesis) intermarried with them. There are traces of what appear to be Neanderthal DNA in modern Europeans that are not found in people in other regions, suggesting that they were simply outnumbered as successive waves of our people kept coming, diluting their contribution to the gene pool.

Many reasons for our supplanting of the Neanderthals in their own homeland have been suggested. One is that, since Neanderthals were physiologically well adapted to cold weather, the global warming at that time made it a better home for our species, which evolved in the tropics. Another is that the sapiens had better technology, but it's not at all clear that the Neanderthals weren't smart enough to use it, if not invent it.

Jean Auel, in her Clan of the Cave Bear series of "prehistorical novels," asserted that the Neanderthals didn't have the brain center and organs for speech so they had to get by with sign language, giving a tremendous advantage to our people; however this was based on thirty year-old science and it's been subsequently determined that the advantage was actually minor or even nonexistent. In any case, we linguists have no evidence to pinpoint the invention of the technology of spoken language any further back than around 10KYA, so nobody knows whether any people were talking to each other in Europe during the time of the Neanderthals.

At any rate, although you can read plenty about the Paleolithic (Early Stone Age) technology of the first tribes of Homo sapiens to set foot in Europe, and a little about their art and the kinds of trash and graves they left behind, you won't find much information about their ethnicity. All we can say for sure is that, like all other non-African people except the indigenous Australians, they were descended from the second wave of migration out of Africa 50KYA.

We don't know how many successive waves of migration of sapiens tribes from Asia to Europe occurred between their first arrival and historical times. The only living descendants of those people are the Basques, and we haven't been able to find a relationship between them and anyone else. So it's possible that descendants of those first modern humans, more than 30,000 years later, were the ones who kicked off the Neolithic Era, the late Stone Age defined by the Agricultural Revolution and the Paradigm Shift from small clans of nomadic hunter-gatherers to larger permanent farming settlements. This would make them the people who built Stonehenge and all the other impressive prehistoric monuments in Europe.

Or it could be that these earliest people were invaded by another tribe from Asia, who brought newer technology and easily marginalized them; this could have happened multiple times. Perhaps as more human remains are found and their DNA is analyzed we'll have a better picture.

The first European people that we know very much about are the Celts. The earliest archeological evidence of Celtic culture in central Europe is from 1200BCE. Obviously they had been there at least a little longer, and equally obviously the journey up from the Indo-European homeland in western Asia took some time. So to date the start of the Indo-European diaspora to approximately 2000BCE is reasonable, although some scholars push it much further back.

By the time Roman historians first mention the Celtic tribes in 400BCE, they inhabited western Europe including the British Isles, most of central Europe including northern Italy, and bits of eastern Europe including Bohemia (named after the Celtic Bohumil) and a colony in Turkey. The Celts did not populate Scandinavia, which was the province of the Germanic tribes we now call the Norse.

The Celtic branch of the Western Indo-European languages is broken into four groups:

• Gallic, named after the Gauls of southern France. The most geographically widespread group, all of these languages are extinct.
• Celtiberian, spoken in what is now Spain and Portugal. Also all extinct.
• Brythonic, the languages of Britannia. These include British, the extinct language of the original Celtic "Britons" who were displaced by the Anglo-Saxon invaders, Welsh, Cornish, Cumbric, and Breton, the language of a Celtic tribe who fled to the continent to escape the Anglo-Saxon invasion.
• Goidelic, including Irish and the languages of the Irish explorers and emigrants, Manx and Scots Gaelic.

The fate of the various Celtic peoples in Europe:

• The Romans colonized Iberia, marginalizing the Celtic tribes and their culture, although they still have bagpipes, known as gaitas in Spanish.
• The Germanic tribes eventually migrated out of Scandinavia through Jutland, marginalizing the Celts in northern central Europe from Holland to Germany.
• The Greeks and the Romans, of course, had already taken over south central and southeastern Europe before either of them had a written language to record the fate of the Celtic tribes there. As the Romans expanded northward they displaced the tribes that the Germans missed.
• However, on their way west they did not obliterate the Gauls. Nonetheless the Gauls were being pushed from the north by the Franks, a Germanic tribe. By the time the Roman Empire disintegrated the Franks had control of the region--the reason we now call it France. But both the Franks and the Gauls had been Romanized and spoke Vulgar Latin, which evolved into French. You can still hear the flapped Celtic R in the French of southern France, versus the gargled Germanic R in Parisian French.
• The Slavic tribes began moving into eastern Europe around 300CE, and that was the end of the Bohumil and any other Celtic tribes on the continent who had managed to evade the Greek, Roman and Germanic invaders.
• When the Roman Empire collapsed around 400CE the only Celtic cultures left in Europe were on the British Isles. However, when the Romans abandoned their outposts on Britannia, Germanic Angles, Saxons and Jutes promptly sailed over and seized the abandoned civilization-in-progress. Unlike the Roman occupation, they completely took over the British nation, marginalizing the Britons, driving them off, or marrying into their families, and remade southern Britannia as "Angle Land."
• The small Celtic communities of Wales, Cornwall, the Isle of Man, and surely a few other places managed to survive this occupation, and as I noted the Bretons established a new outpost, Brittany, on the French Coast. But the only major Celtic nation in the middle of the first millennium CE was Ireland: the remnant of the people who once populated virtually all of sub-Scandinavian Europe.
• Around 700CE the Irish changed that by sending explorers to northern Britannia. There were people living there whom we know very little about, called the Picts. Some scholars say they were a Celtic tribe, some say they were the last pre-Indo-European people and may have been the same tribe who built Stonehenge, while others admit honestly that they just don't know.

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  In any case the Irish invaders/occupiers/colonists came to dominate the region and established their own culture and language (Gaelic) there.
• The Roman name for the Irish people was Scoti, so the northern part of Britannia came to be known as Scotia in Latin or Scotland in English. The Irish people back in Ireland hung onto their own Irish name for themselves, "Irish," so the people of Scotland came to be known as Scots. The Irish and the Scots speak two intercomprehensible dialects of Gaelic, but for historical and political reasons the Irish generally prefer to call their tongue Irish.
• As we all know the history of Scotland since then has been tumultuous, but eventually it united with England (and Wales, which had already been partnered, and Cornwall, which was actually annexed to England). Today all Scots speak English and many don't speak Scots Gaelic, although the language is undergoing a resurgence. But their Celtic culture is strong and quite evident in other ways.

So three Celtic nations remain: Ireland, Scotland and Wales. And Celtic culture and language are also hanging on in Cornwall and Brittany, although both are now parts of other countries, England and France.