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From a German user perspective, ICU and your fancy library are incorrect, actually. Mass is not a different casing of Maß, they are different characters. Google likely changed this because it didn't do what users wanted.

It isn't until it is, how would you write it when ß isn't available on the keyboard?

Which is why we also have to deal with the ue, ae, oe kind of trick, also known as Ersatzschreibweise.

Then German language users from de-CH region, consider Mass the correct way.

Yeah, localization and internalization is a mess to get right.

Case insensitivity is localized like anything else. I and i are equivalent, right? Not if you’re doing Turkish, then it’s I and ı, and İ and i.

In practice you can do pretty well with a universal approach, but it can’t be 100% correct.

This is a very good example! Still, “correct” needs context. You can be 100% “correct with respect to ICU”. It’s definitely not perfect, but it’s the best standard we have. And luckily for me, it also defines the locale-independent rules. I can expand to support locale-specific adjustments in the future, but waiting for the adoption to grow before investing even more engineering effort into this feature. Maybe worth opening a GitHub issue for that :)