No de_DE Keyboard-Layout? Lazy? This is a additional de_umlauts.utf-8.add spellfile to add Umlauts ÄäÖöÜüß in their long form (i.e. Ae, oe or ss) within vim.
To add cat de_umlauts.utf-8.add >> ~/.vim/spell/de.utf-8.add
Apply in vim (once): :mkspell! ~/.vim/spell/de.utf-8.add