In QLocale::uiLanguages() (which, at ~250LOCs, is way too long, btw), there are several problematic operations on QStringList uiLanguages. The most obvious is the manual re-implementation of QStringList::removeDuplicates() as a quadratic instead of linear algorithm. This is easily solved by using the Real Thing™.

But in the addIfEquivalent(), the {{insert()}}s are done in the middle, too, and it's not clear whether they're all done near the end, where it would just be "meh", but not actually cause a complexity problem. In the worst case, though, the list is built back to front, which would make the construction have quadratic complexity, too (unless prepend optimization kicks in for in-the-middle-but-near-the-front insertions, too; tbc).

Ideally, the list would be build with only appends, iow: insertions are done in order of precedence. I haven't done enough analysis to determine whether that's feasible.