The Iberian Peninsula was made up of many independent fiefdoms and principalities, each with its own language, customs and history. Spain was made up of many of these little dukedoms and petty kingdoms, such as Castile, Extremadura, Aragon, Leon, Catalonia, Asturias, etc. And half of modern Spain was controlled by the Moors for centuries.

This pattern was not unlike the rest of Europe. France had Normandy, Burgundy, Alsace, Lorraine, Aquitaine, Provence, etc, and Britain had Essex, Wessex, Sussex, the Danelaw, Cornwall, not to mention the neighboring cultural and linguistically independent kingdoms of Scotland, Wales and Ireland. I think the last native speaker of Cornish died in the 1920s.

Many European countries, like Italy and Germany, weren't fully unified into nations until the late nineteeth century. Some of these medieval "countries" survive today as provinces, (think Lombardy, Tuscany, Sicily, Piedmonte, Calabria) and many have strong nationalistic and even separatist movements, their own national dialects, costume, culture, poets, and so on. I guess after Rome fell and the Empire's road network and security arrangements fell into disrepair, it became difficult for the local barons to control an area larger than the distance a man on horseback could cover in a few days.

In Spain, the one I'm most familiar with, Castile and Leon (Ferdinand and Isabella) became the dominant kingdoms after kicking out the Moors in 1492, and Castilian the dominant language. I speak the New World variant of Castilian, Cuban Coastal Dialect, specifically, although I can still make myself understood in Madrid if I had to. In Catalonia, there is a strong nationalist and cultural movement that is resurging. I patrol their websites occasionally, and I can barely even read their script. I had a relative (by marriage, now deceased), who was celebrated as the leading bagpipe player of Asturias. Yes, Asturias is a Celtic enclave in Spain, as is Brittany in France, they wear kilts and play the pipes there, and still have pockets of Gaelic speakers. In the Basque country around Barcelona, they even speak a language that isn't remotely related to any European tongue. No one's quite sure where it came from, there is no other other related language on earth.

Many of these ancient cultural boundaries are still mappable, modern nation-state boundaries have ebbed and flowed across them. For example, I can understand a little Portuguese and Provencal, because of their nearness to Spain. And German, French, Italian and Romanch are spoken in Switzerland. Across the channel from the UK a dialect of Frisian is spoken that sounds a great deal like English, and shares many common features with it.

America is a recent invention, with no long-term history to speak of, so this kind of cultural richness is difficult for us to comprehend. I guess the closest we ever came to it was in ante-bellum America, where people still identified with their home State, not the United States. I can tell the difference between a Southerner, a New Englander and a Midwesterner, by both custom and speech, but to my grandfather they were indistinguishable.

Americans have trouble with other cultures, they have difficulty visualizing why people from other countries don't think like they do, and insist on calling this cultural naivete "exceptionalism". We've only been exposed to one culture for most of our history, and we have trouble coming to grips with the idea that the Nation-State as we know it is a relatively recent historical invention. It first appeared about 400 years ago and didn't become common in Europe until the 19th century. According to the Wiki Article on the Nation-state:

"In France, Eric Hobsbawm argues the French state preceded the formation of the French people. Hobsbawm considers that the state made the French nation, not French nationalism, which emerged at the end of the 19th century, the time of the Dreyfus Affair. At the time of the 1789 French Revolution, only half of the French people spoke some French, and 12-13% spoke it "fairly", according to Hobsbawm.

"During the Italian unification, the number of people speaking the Italian language was even lower. The French state promoted the unification of various dialects and languages into the French language. The introduction of conscription and the Third Republic's 1880s laws on public instruction, facilitated the creation of a national identity, under this theory."

I had a friend in college who emigrated from northern Italy to Venezuela when he was 11 years old. He spoke bad English, bad Spanish, bad Italian, and was only fluent in the dialect of his village in the Italian Alps, a place so remote no one there had ever seen the beverage "tea" until after the First World War.

BTW, Portugal was the first great European seafaring nation. They developed advance navigational techniques and a very seaworthy ocean going craft, the lateen-rigged caravel, that allowed them to sail around the horn of Africa and open profitable colonies in Africa (Angola and Mozambique), India (Goa and Ceylon), Indonesia, China (Macao) and Japan, finally breaking the Arab monopoly of the Silk Road trade. Columbus sailied West towards the Indies under Spanish colors because Portugal controlled the South Atlantic and Indian Oceans.