Why can we use Ü in character names but not Ä or Ö?

Ää Öö and Üü are letters used in a few languages like Finnish, German or Turkish. I understand that most games do not support these letters, but what confuses me is that in PoE you can use the letter Ü in a character's name, but not Ä or Ö.

If we can have Ü, why not the other two as well?