EU doesn't believe in nation states, only regions.
How many "regions" did EU make out of Yugoslavia?
The various countries in Yugoslavia separated pretty much on their own
The reason is quite different from Spain -- the Kingdom of the Southern Slavs was an artificial construct -- these had never been one nation before 1918 and they had had divergent histories for nearly a millenia
the Serbs, Macedonians and Montenegrans are Orthodox, the Bosnians and Kosovar Albanians are Moslems, the Croats and Slovenes are Catholic (more in case of the Croats, less in case of the Slovenes) and there are/were minor Hungarian, Saxon, Turk and Roma gypsy minorities along with Greeks and Jews and Armenians
there is a lot of speculation about Croat and Serb histories -- some say the Croats are Catholic Serbs, others that Serbs are Orthodox Croats and others that say that there were separate groups right from the 8th century - the white Croats (descended from Sarmatians) and white Serbs (who initially lived in the Vistula valley)
In my opinion, both are separate groupings of tribes that coalesced in the 10th century from the various South Slavic peoples (who separated from the West and East Slavs in the 9th century and were definitely cut off from them by the invasions of the Magyar)
Ditto for the Bosnians -- many Serbs and Croats say they are descendents of Turks, but my opinion is that these are primarily Slavs who converted. Perhaps they were Bogumils (a Gnostic Christian sect) or one stuck between Rome and Constantinople which is why they converted, or maybe they just converted under pressure or otherwise. But they are Slavic primarily
The Albanians are not Slavs but a different Indo-European people (lots of arguments here and there)
Anyway, the Croats were firmly part of the West right from the 12th century and in the 13th they joined with Hungary and this union was fixed until 1918.
The Serbs had a divergent path -- initially part of the Bulgar Empire (founded by Bulgars whose elite were Turkic but who became thoroughly slavicized by the 8th century) and then formed their own Tsardom which was then shattered by the Ottoman Turks)
they were heavily influenced by Church Slavonic and Eastern Orthodoxy and by their struggles with the Turks
The Bosnians may have just been converts who were despised by those they left behind (because in the Ottoman system, if you converted to Islam, you became part of the Turkic millet)
As an aside the language these 3 speak is really the same, Serbo-Croat, with dialects for each (and getting increasingly separated with Croats adding Latin words, Bosnians adding Arabic/Turkic and Serbs Church Slavonic) and they are part of the wider South-Slavic spectrum of languages from Bulgarian to Slovenian.
Bulgarian and Slovenian are somewhat mutually intelligible, but Bulgarian and Serbo-Croat is pretty mutually intelligible
The Montenegrans are Serbs in religion and ethnicity, separated only in the 1800s due to political winds (they are "mountain Serbs" who the Turks could never quite conquer) and this was cemented in 1918
The "Macedonians" are closer to Bulgarians than anything with their language being very Bulgarian-like, more than Serb-like.
the Slovenes have many Italic and Allemanic Germanic (through Austria) words -- they were separated from Croatia because they were part of the Austrian half of Austro-Hungaria while the Croats were part of the Hungarian half
All of this separation meant that the different groups wanted to retain their differences -- but the artificial construct of Yugoslavia in 1918 made many fear domination by one or the other
The Croats under the Ustashe committed unspeakable horrors against their brethren the Serbs -- fratricide which I can't understand -- utterly despicable. And that has poisoned Serb-Croat relations since then
Tito, a Croat, managed to bottle these ethnic tensions under "communism for all", crushing all ethnic sentiments
But this only postponed the bloodshed
The sins of the Ustashe and of the Turks had to be paid and the Serbs went on their own set of dominance
I'm not going to side any one side for the croat-serb-bosnian conflict just to say that it wasn't all the serbs fault, and we shouldn't have bombed them.
What else we should have done, I don't know
The Catalan language is distinct, but closer to Castillian
Historically there has been no centuries old war between the Catalans and the Castlinians (note: I said centuries old)
Catalan was the first "March" set up by Charlemagne to fight back the Moors and it was incorporated into the Kingdom of Aragon
in contrast to the west, the Kingdom of Castile encompassed Galicia and moved south into Extramudura
the Aragon lands had more Moslems than the Castilians and they tended to be more "open" to differences than the Castilians. But note that this was Aragon (which included Catalan)
The ARagon kingdom then expanded to form the Aragon Empire consisting of Sardinia and then the kingdom of the Two Sicilies (the island of Sicily proper and then the boot of Italy) and parts of Greece and Rhodes
They joined with Castile in the grand union of 1487 and then completed the Reconquista, taking over the emirate of Granada
No antagonism so far and this continued until the war of Spanish succession.
The Catalans supported the Hapsburg candidate and they were opposed by the French who wanted to put the Bourbon (same family as the French king) on the throne. The Bourbon's won
But the real antagonism was during the spanish civil war when Catalan was heavily influenced by anti-clerical socialism (well, communism) and fought against Franco
Even today there isn't antagonism, because Spain is a VERY decentralized country -- each state has its own flag, nearly separate government etc. -- more federal than Germany and nothing like France (which is heavily centralized in Paris)
And that's not completely true.
The EU is dominated by France and Germany -- and France would not stand for "regions" -- they are so centralized they even refuse to acknowledge distinct language groups like in Brittany
Germany in contrast even recognizes the Sorbians
the "regions" are more theoretical than actual -- and in one case it has helped -- the Tyrol region: South Tyrol was part of Austria until 1918 when it was handed over to Italy even though the majority of its population was German speaking
This was defused by forming a cross-border region where people could speak both languages in governance and had local government
It would also help in the case of the Basque region