Even the accented letters are a problem. Accented letters often come from Western or Nothern European countries. Which all use the ISO LATIN-1 alphabet. But while an accented letter may look the same in different countries, they are different. In some languages, an accent just means the letter is pronounced differently, but it's still the same letter. But the same accent can change the letter in a different language. Which will become a different letter. And even if you have two languages who use the same accented letter, it doesn't necessarely mean they the letters sort the same.
Which is why we have locales. And which means that whatever solution you will pick - there are people that will be surprised.
If only we all spoke (and wrote) Egyptian hieroglyphs, we would have this mess.