The diaeresis in this context is not an Umlaut. It has similar roots as on ï which comes from French, I think. While äöü in German are letters with distinct sou

The diaeresis in this context is not an Umlaut. It has similar roots as on ï which comes from French, I think. While äöü in German are letters with distinct sounds, the diaeresis on vowels in English usage signifies a short pause before that vowel.