If you exclude German pages, I see less than a dozen occurrences of this on English pages. Of those, all but a few put it in quotes and then explain it.

If you search English pages for Bauhaus (372,000) Jugendstil (38,800), De Stijl (40,100) or Art Nouveau (929,000) you could argue (as I would) that these are English words. But 'Zopfstil', not yet.