Mate, this question comes up from time to time, and it's been answered several times. You might try searching past answers in case mine isn't sufficient. Basically, reactions varied from country to country as well as within countries, with most perspectives favoring the Union and a minority of conservative Europeans favoring the Confederacy.

Here's a piece from the Nov. 29, 1860 London Times, reprinted Dec. 15, 1860 in the New York Times. The key feature is this:

"Can any sane man believe that England and France will consent, as is now suggested, to stultify the policy of half a century for the sake of an extended cotton trade, and to purchase the favor of Charleston and Milledgeville by recognizing what has been called "the isothermal law, which impels African labor towards the tropics" on the other side of the Atlantic?"

Britain and France sent observers to monitor the war, as did Prussia. As much as we talk about the cotton trade's importance to Britain, modern scholarship has revealed the extent to which Britain depended on the grain trade, using it as a tool to explain that while British merchants and shipbuilders were happy to sell to the Confederacy, the British government was unlikely to intervene despite intense diplomatic effort from the Confederacy.

While the French government was inclined to intervene, Napoleon III would not do so without the support of the British Empire. Even if he wouldn't intervene directly in the war, he still saw opportunity.

Spain, France and Britain took advantage of the Union's preoccupation to push forward colonial ambitions in the Western Hemisphere. France invaded Mexico. Spain landed in the Dominican Republic. In Cuba, even as Spain maintained its hold, slaves sang "Avanza, Lincoln, avanza! Tu eres nuestra esperanza!” (Onward, Lincoln, Onward! You are our hope!), viewing, as many in Europe did, that this was a fight between slavery and liberation.

Indeed, many Europeans saw this fact before Americans themselves did. Lincoln and many in the North saw the war in 1861 as one to preserve the Union. Only as the fighting progressed did it become a matter of emancipation and justice.

The Italian liberator and general Garibaldi twice was encouraged to come to the United States and fight on the side of the Union army. "Tell me if this agitation is regarding the emancipation of the Negroes or not," he asked in 1861, then declined.

Prussia heavily favored the Union cause. After the failed revolutions of 1848, hundreds of thousands of Germans emigrated to the United States. They brought a strong work ethic and the progressive ideals of central Europe. These included notions like universal free public education, human rights, and emancipation.

When the American Civil War broke out, Germans flocked to volunteer in huge numbers. Of all the immigrant populations in the United States, only the Irish contributed more soldiers to the Union. More than just weight of arms, the Germans were among the first to believe that the war was about abolishing slavery as much as it was preserving the Union.

There were German officers as well, commanding predominantly German-speaking units. Franz Siegel is perhaps the most famous, but there many men like Col. Ernst von Vegesack ─ in reality a Swedish nobleman, he shouted Bahn Frei! as the 20th New York (mostly German) streamed into battle at Antietam.

Behind the lines, there were men like Francis Lieber, who I've written about before. Lieber, already famous as an intellectual at the outbreak of war, used liberal German values to help formulate laws of war for the Union Army. He strongly pressured Lincoln in favor of the Emancipation Proclamation.

With so many Germans in the Union Army, it's no wonder that popular opinion in Germany favored the United States. You're more likely to appreciate and cheer for the side that most closely resembles your own, after all. Furthermore, the Union appeared ─ particularly later in the war, and particularly among European revolutionaries ─ to be fighting to end slavery, even if American diplomats and most Americans would not admit that until the last two years of the war. That was an incredibly popular aim, and particularly so in Germany.

Now, keep in mind the acclaim for the United States wasn't universal. If you're interested in reading a contemporary account of the American Civil War in German, find the writings of Justus Scheibert. He was a Prussian observer who sneaked into Charleston aboard a blockade runner in 1863 and proceeded to write about the war. He took a strongly pro-Confederate approach, believing the South had elite origins, just as Prussia did.

To counter that, you may want to consider some of the other works edited by Frederic Trautmann, including the regimental history of the all-German 9th Ohio Infantry and the prison memoirs of Bernhard Domschcke.

Russia had long supported the United States, and especially after the Crimean War viewed it as a potential counterweight to Great Britain. During the war scare of 1863, when it appeared that French and British intervention in Poland was a possibility, Russia sent large components of its Atlantic and Pacific fleets to the United States as a precaution against having them trapped in Russian ports if war broke out. Americans viewed this as the first step toward Russian support of the war ─ it wasn't.

Volunteers from almost every country in Europe could be found in the Union ranks, though Germans and Irish get the top billing in popular history. There was the 55th New York Infantry, for example. That regiment's original recruits came from French émigrés in New York City. Even by January 1862, six of the regiment's nine companies were predominantly French. As James Johnston wrote in 2012, "Some of the men were veterans, having served in the French Army in Algeria, the Crimean War and Italy. The rest were a motley international bunch, including German, Irish, Italian and Spanish immigrants, as well as a few Americans."

To add on to this question, how did non-European nations react, like nations in Latin America? I'm particularly interested if Japan or China had anything to say about it (since the Japanese had already been contacted by the US by Commodore Perry, and the US was trading with Qing China).

If you're interested in this topic, there's a book I've reccomended a few times here before: The Cause of All Nations, by Don Doyle. It covers exactly the topic you asked about, focusing especially on Britain and France but also covering the reaction to the Civil War in other European nations.