In contrast, languages like German and the Scandinavian languages treat accented vowels as distinct vowels of their own, moving them to the end of the alphabet in the dictionary.

In German, the umlaut is analysed alphabetically as a digraph of the base vowel plus “e”, which is its ultimate source. The symbol became a diacritic mark through reduction of an “e” written above the vowel. I believe you’re correct about Scandinavian languages though.