One reaction might seem a bit strange without context--the pope declared himself a "prisoner of the Vatican".

The ongoing conflict over Italian unification resulted in the Papal States being reduced to Rome and its environs by 1860-61. The only things that kept Italian forces from taking Rome were a bit of concern over not attacking the pontiff directly and a contingent of French troops. Then the Franco-Prussian war resulted in a French collapse during which the French troops were recalled. The Italians overcame their remaining reluctance and captured the rest of Rome with the exception of the Vatican itself.

Italy finally held Rome, the pontiff retreated behind the Vatican's walls, and the new Italian state and the papacy settled into an uneasy staring match that negotiations never quite resolved. This situation--The Roman Question--continued until 1929 when the Lateran Accords created the new nation of Vatican City and relations were normalized between Italy and the Vatican. So, the Franco-Prussian war allowed Italy to claim Rome but sparked a continuing diplomatic dispute that lasted well into the 20th century.

The sieges/battles at Metz, Strasbourg, and Paris were certainly very brutal, and Prussia even committed some "war crimes" by firing upon civilian buildings and hospitals. In the last few weeks of the war, French diplomats tried to attract the attention of third parties so that Bismarck would spare Paris. The French believed that no one would allow Prussia to besiege a city like Paris, which was considered by many to be the cultural capital of Europe. Bismarck himself spent a lot of time in Paris in the decade before the war!

Jules Favre, the Minister of Foreign Affairs, requested aid from neighboring countries without any responses. Elihu Washburne, the American ambassador to France, negotiated relief efforts with Bismarck - also with little to show for it. People were willing to let it play its course.

That being said, many countries were sympathetic to France. The government that had declared war, the Second Empire, fell with the Capture of Napoleon at Sedan. Many believed - including some moderate republicans - that Prussia would cease the war since France was now a Republic. The Republican élan would have possibly convinced others to intervene, but unfortunately, the revolution tended to galvanize the French into resistance against the enemy. The terminology of the time called for a "guerre à outrance" or an all-out-war. This certainly did not help win them any sympathies from the outside.

Yes, Germany was excessive in their celebrations. Sedantag - the anniversary of their razing of Sedan and the capture of Napoleon - was commemorated in Germany until the First World War. The unification of Germany was signed in Versailles. Germans unnecessarily insisted that they occupy Paris after its capitulation. But the French were just as hubristic and enthusiastic at the outset of hostilities. The French believed they would capture Berlin in a matter of weeks; of course, it was the complete opposite. The cultures of overconfident boasting was most likely what pushed Germany to humiliate the French after their defeat. One German contemporary said

“A novel element has arisen in politics, an additional level, of which earlier victors knew nothing or at least made no conscious use. One seeks to humiliate the vanquished enemy as much as possible in his own eyes so that he henceforth lacks any confidence whatsoever.”

I got a little off topic there, but the point is that despite the "novel element," foreign countries were unwilling to intervene. For more reading, take a look at Rachel Chrastil's "Organizing for War: France 1870-1914", "Elihu Washburne" (his correspondence and diary during the war), edited by Michael Hill, or Wolfgang Schivelbusch's "The Culture of Defeat."

For Britain, her traditional, hereditary enemy had been humbled. For many in people in Britain, the vulgar bombast of Napoleon III and the tawdry glitz of his Empire had been exposed by Prussia. Don't forget that in the 1850s and 60s there were invasion scares concerning the possibility of France's steam-powered ships being able to transport tens of thousands of soldiers across the Channel over night. Many people believed that French imperialism had caused the war.

However Disraeli's reaction deserves quotation as a remarkably prescient appreciation of what had happened:

Let me impress upon the attention of the House the character of this war between France and Germany. It is no common war, like the war between Prussia and Austria, or like the Italian war in which France was engaged some years ago; nor is it like the Crimean War. This war represents the German revolution, a greater political event than the French revolution of last century. I don't say a greater, or as great a social event. What its social consequences may be are in the future. Not a single principle in the management of our foreign affairs, accepted by all statesmen for guidance up to six months ago, any longer exists. There is not a diplomatic tradition which has not been swept away. You have a new world, new influences at work, new and unknown objects and dangers with which to cope, at present involved in that obscurity incident to novelty in such affairs. We used to have discussions in this House about the balance of power. Lord Palmerston, eminently a practical man, trimmed the ship of State and shaped its policy with a view to preserve an equilibrium in Europe. ... But what has really come to pass? The balance of power has been entirely destroyed, and the country which suffers most, and feels the effects of this great change, is England.

What you also see happening in Britain is the rise of invasion literature, (The Battle of Dorking is the most famous). Not all of them portrayed Germany as the invader (sometimes it was France) but increasingly the older view of Germany as a land of culture (Goethe etc.) is superseded by image of Germany as dominated by Prussian militarism and a reactionary Junker class.