Things like text search and exact string matching.
As minor as it might sound, if 199/200 countries work in your system, capitalism dictates that below a certain business size, the logical thing is often to just ignore the one country and move on.
I have good news for you - text search for non-ascii version of latin characters like ü is a long solved problem. That's because even in non-english speaking countries some people don't bother with diacritics, or they don't have a correct layout at that point, or the text is in source code and has to be ASCII, etc.
You can try that now - my browser highlights Türkiye when I search for Turkiye.
That's not a keyboard issue. Everyone in my country uses a standard US keyboard, we just press AlgGr to type some characters. I even learned some foreign lanugages, so I use a (standard, included by default in every linux distro) keyboard layout that allows me to type most characters used in European languages by using chords - for example, alt-shift-2 + u = ü.
So this is not a keyboard (hardware) problem, just a keyboard layout (software) problem.
It's the right-alt key. If you're using a layout that defines AltGr, the key will just work like that on a US keyboard. Even with just the US keyboard layout, the right-Alt key can still be used with numbers in Windows to type any unicode character.
Which the parent was pointing out is nonstandard. There is no alt-gr key on a standard US keyboard. You can certainly remap alt-right to alt-gr but that is nonstandard and doing so does not mean that the key exists on the physical keyboard.
For example my caps lock key is mapped to ctrl-left. That is nonstandard.
AFAIK neither Windows nor X11 have a concept of an AltGr key - it's just the right alt key so there is nothing to remap. The only difference between US and european layouts is its use as a modifier key.