Also note that 'are these 2 words case insensitively equal' is impossible without knowing what locale rules to apply. And given that people's personal names tend to have the property that any locale rules that must be applied are _the locale that their name originates from_, and that no repository of names I am aware of stores locale along with the name, that means what you want, is impossible.

In line with case insensitivity, do you think `müller` and `muller` should boil down to for example the same username for login purposes?

That's... tricky. In german, the standard way to transliterate names to strict ASCII would be to turn `müller` into `mueller`. In swiss german that is in fact mandatory. Nobody in switserland is named `müller` but you'll find loads of `mueller`s. Except.. there _are_ `müller` in switzerland - probably german citizens living ther.

So, just normalize `ü` to `ue`, easy, right? Except that one doesn't reverse all that well, but that's probably allright. But - no. In other locales, the asciification of `ü` is not `ue`. For example, `Sjögren` is swedish and that transliterates to `sjogren`, not `sjoegren`.

Bringing it back to casing: Given the string `IJSSELMEER`, if I want to title case that, the correct output is presumably `IJsselmeer`. Yes, that's an intentional capital I capital J. Because it's a dutch word and that's how it goes. In an optimal world, there is a separate unicode glyph for the dutch IJ as a single letter so we can stick with the simple rule of 'to title case a string, upper case the first glyph and lowercase all others, until you see a space glyph, in which case, uppercase the next'. But the dutch were using computers fairly early on and went with using the I and the J (plain ascii) for this stuff.

And then we get into well trodden ground: In turkish, there is both a dotted and a dotless i. For... reasons they use plain jane ascii `i` for lowercase dotted i and plain jane ascii `I` for uppercase dotless I. But they have fancy non-ascii unicode glyphs for 'dotted capital I' and 'dotless lowercase i'.

So, __in turkish__, `IZMIR` is not case-insensitive equal to `izmir`. Instead, `İZMIR` and `izmir` are equal.

I don't know how to solve this without either bringing in hard AI (as in, a system that recognizes 'müller' as a common german surname and treats it as equal to 'mueller', but it would not treat `xyzmü` equal to `xyzmue` - and treats IZMIR as not equal to izmir, because it recognizes it as the name of a major turkish city and thus applies turkish locale rules), or decreeing to the internet: "get lost with your fancypants non-US/UKian weird word stuff. Fix your language or something" - which, well, most cultures aren't going to like.

'files are case insensitive' sidesteps alllllll of this.