He was Swiss, more exactly from the city of Winterthur located in the canton (state) of Zürich. The canton's official language is German, however. Of course, people over there speak in a strong local dialect called "Züritüütsch".
You just elongate the vowel i.e. pronounce it longer. The double „ü“ just indicates this vowel to be stressed. Dialects do not follow a strict orthography, however, so you might find it written slightly differently in other contexts.
Wirth lived in the United States for some time throughout his life but is a Zürich native. He must have spoken Züritüütsch („Zurich German“) privately, I am pretty sure (without having known him personally).
Umlauts aren't diphtongs; it's the same sound all the way through. GP used two consecutive ones in order to show that the sound is long. (And whaddoino, if the dialect has an official orthography, maybe that's how it's supposed to be spelled.)
He was Swiss, more exactly from the city of Winterthur located in the canton (state) of Zürich. The canton's official language is German, however. Of course, people over there speak in a strong local dialect called "Züritüütsch".