The standard does not specify how one should alphabetize the letter "ü" when used in other languages than German, but at least as regards the Estonian or Hungarian "ü", it seems consistent to treat it as equivalent to "y" (and even more so, since "ü" in Estonian and Hungarian is not considered a mere variant of "u", as it is in German). It would seem problematic, however, to apply the same principle to e.g. "ü" (u-diaeresis) as used in Spanish or "õ" (nasal vowel) as used in Portuguese, as these letters represent quite different orthographic traditions.