What's the consensus on the acceptability (or lack thereof) of using the term "Victorian" for the history of non-British countries from 1837 to 1901? Is it considered bad practice to speak of "the Victorian U.S."? What about, say, "Victorian Spain" or "Victorian France"?

I have only ever heard the term used referring to the United Kingdom, and in a limited sense with the United States when discussing specific artistic, cultural, and social ideas linking the anglosphere. In other countries where Queen Victoria was not monarch nor a disproportionately influential figure, periodization of the nineteenth century can indeed be different. In my own area of interest, Italy, the century is generally divided into four periods:

L'Italia Napoleonica: understandably, "Napoleonic Italy," as one would refer to "Napoleonic France." This period ends not with Napoleon's defeat at Waterloo, but with his defeat following the War of the Sixth Coalition in 1814 after which the Italian states were partially restored.

La Restaurazione: that is to say, "The Restoration," which is also a term used widely in the rest of continental western Europe.

Il Risorgimento: that is to say, the period of activism, social unrest, and war that brought about Italian Unification. Starting sometime in the 1830s, there is certainly some overlap with the Restaurazione, seeing as the Risorgimento was in great part a reaction against the post-Napoleonic order (however, there are few periodization that are not problematic with regards to clear beginning and ending dates). Architecturally, intellectually, and artistically, this period is largely aligned with the esthetics of the Second French Empire, however "unitarist" themes are a recurring thread; with, for example, Venetian-inspired architecture being built in Milan, and allegoric works about unity and a lost fatherland appearing in literature and the performing arts.

Il Periodo Unitario: generally understood to mean the period following the seizure of Rome by Italian forces in 1870, however some might argue it starts with the proclamation of the Kingdom of Italy in 1861, with the capital in Florence. Because so much of the political and social dialogue of the time dealt with the construction of the new Italian state, it is common to refer to things in this period as belonging to "L'Italia Unitaria," emphasizing the new unity. Artistically, especially architecturally, the period is generally aligned with Europe's gradual embrace of the Art Nouveau movement, however its Italian incarnation is more commonly denominated "Stile Liberty."