The international/European response is not set in stone and could wildly vary based on the Spanish response. If they send in troops, and should those troops fire on civilians, the Spanish may well lose international support. They were heavily condemned just for their police actions around the referendum.

There also aren't that many countries that have serious internal strife in the western world anyway, that point is always wildly overstated. The UK has independence movements but they've already allowed a referendum so they may not care. Belgium and Russia have some serious internal trouble, but the importance of their support may be too small to matter (and indeed Russia may encourage discord). France and Italy? But are their minorities really serious enough? What else is there? The vast majority of Europe will swing towards what has the best optics, as will the US. Sure they may pick stability first, but if that's out because of civil war, it becomes anyones guess.