The ASCII conversion for slugs is not so SEO-friendly as it could.
For example, “Nos Œuvres Cachées” could produce nos-oeuvres-cachees (ASCII) but actually produce nos-uvres-cachees, which breaks words.
An UTF-8 character is automatically replaced with a -, which often breaks in non-English languages (e.g., in French, German, and other languages with lots of accents). In German, the umlaut (e.g., ü) is traditionally replaced with ue in URLs.
(Feel free to move this post to a new topic.)