There are 23 official languages within the European Union and many, if not all, of them have special characters. In German, for example, there are Umlauts (“üöä”) and the “ß”; and in other languages, there are more. Many characters exist in a pre-composed version and as a combination of two characters. Using the two-character version can lead to a broken search, broken spell check, broken transliteration for the slug, and broken images if this happens in a filename in combination with some server configuration and browsers.
Presentation from WordCamp Europe 2019.