Earlier quoted context omitted.
It's a circular dependency: how do you sort and list the locales or languages for someone to pick one, when by definition you don't know their locale yet? You have to either make some best-guess approximation (IP geo, browser headers, etc) or use a locale-invariant sort, both of which will be wrong in some cases.
We can find a sorting order with the minimal total distance between where we place a language entry and where this entry would be in that language. If there’s no pair of languages A and Ä such that A > Ä in one and Ä > A in the other, then (I guess???) this total distance will be zero.
Coincidentally, the expected position of "Ä" can vary wildly. Is it an umlauted A, normalized as AE, or a distinct letter coming after Z?