My apologies, but when I think about regions within the vaguely same cultural group unifying, I think of Europe, with the formation of Italy, Germany, and the stomping out of local linguistic groups in France and Spain in the 19th century. I'm sure other parts of the Old World (I don't really think this happens in the Americas) did the same, but I can't think of anything similar.
Small Nitpick: You mean 20th century, between 1936 and 1975, and in a totally different historical contest to what you have in mind. The language situation in 19th century Spain was far closer to that of Austria than France - this is a time of rebirth of local languages, not "stomping them out".
Now to the main question: You sort of answered yourself.
The thing is, language is a great factor of consolidation within an existing state, as is religion,
after the regions in question have been put within that same state,
but the mere existence of a common or similar language, or religion, or even shared history between two neighbouring regions doesn't constitute an actual driving force towards unification despite what that brief spark of 19th century romanticism could lead us to think.
And yes, it's no different in Europe or in America. If language was everything needed for political unification, Germany would include Austria, Luxembourg, Liechtenstein and a chunk of Switzerland, and there would be a single state from Tijuana to Cape Horn. So what prevents that from happening?
Long aswer: A conjunction of factors including random circunstances, internal and foreign politics, and yep, geography, since that will determine communications, climate, fertility of soil, mineral resources and in the end the ability of X region to raise a bigger army than Y region and send there successfully. Short answer: What, in essence, we call
History.
...Woah. The way that thread unfolded was completely ridiculous.
Well, geography maybe the reason why the Occitan-Catalan sphere didn't unite.
More specifically, it was Peter II of Aragon being handed his ass on a plate at Muret, and his son James deciding to never fuck with France again after spending his childhood in French captivity and deciding to deploy his forces southwards instead.
Uff Da the optimist said:
Why exactly didn't Portugal get absorbed with the rest of the Iberian kingdoms though? They weren't much different in terms of having a slightly different language than Castille, Aragon and Catalonia?
Dynastic union came too late, at a time Portugal had already a colonial empire of its own, and even things could have turned very different if the Spanish had not have to deal with the French invading Catalonia in the 1640s when the Portuguese revolted.