I understand that Physics Stack Exchange "is an English language site", but I'm wondering whether it's acceptable to provide a generic reference (such as a link) to a source, written in a language other than English, whose translation would be very time-consuming. Would its acceptability be general, or would it depend on its being an official language of one or more of the countries whose "official" languages (acceptable, without interpretation necessarily being required, in the civil courts of at least some of their loclities) include English, or would it not be acceptable at all?
Yes, while the language of the main body of a SE post should be in English, non-English references are acceptable for various reasons:
A non-English reference is better than no reference.
It is unfair to not recognize a scientific contribution just because it is not written in English.
The answers on Stack Exchange should be fully complete on their own. Any external references, links, etc. should just be used as supplement. So really it doesn't matter what language your sources are in as long as your post on this site is in English and is complete on its own. One key reason for this is to cover the case for when the reference becomes no longer accessible for any reason (language barrier, I suppose, could be one of those reasons).
Top scientific journals not only accept references in foreign languages, but sometimes even require that such a reference be provided - the well-known case is JETP/ZhETF, which was an ongoing direct translation of the major journal published in Russian.
I think it is in the interests of the community to provide the original sources, as well as, whenever possible, the equivalent sources in English. The former assures that interesting research in not discriminated against, just because of its original language, wheres the latter assures that it is made accessible to broader audience.
It is also worth taking into account that many (or perhaps most) of the members of this community are non-native English speakers.
Finally, I would like to quote "The Little Prince" by Antoine de Saint-Exupery (available in many languages):
This asteroid has only once been seen through the telescope. That was by a Turkish astronomer, in 1909. On making his discovery, the astronomer had presented it to the International Astronomical Congress, in a great demonstration. But he was in Turkish costume, and so nobody would believe what he said. Grown-ups are like that . . . Fortunately, however, for the reputation of Asteroid B-612, a Turkish dictator made a law that his subjects, under pain of death, should change to European costume. So in 1920 the astronomer gave his demonstration all over again, dressed with impressive style and elegance. And this time everybody accepted his report.