Umlaute with stemmer de

[getriebesand]

Describe the bug
"grüne" will be "grun" with the german stemmer. But it must be "grün"

To Reproduce

const natural = require('natural');
const stemmer = natural.PorterStemmerDe;
console.log(stemmer.stem("grüne"));

Output: grun

Expected behavior

Output grün