Wth è questa lettera 😭 ï

Not a German letter

It’s called a Trema and indicates pronunciation: it’s not ‘Seir’ but ‘Se-ir’.

It is not an official letter in German but it is used in several languages to indicate that e an i are not to be pronounced as diphthong ei but as two distinct sounds

That is a Latin letter I with dieresis.

The “two dots diacritic”, also called a “trema”, has at least two functions. In German it usually represents an umlaut, a type of sound change; but in some other languages, and sometimes also in German, it can represent a diaeresis — this is when two vowels that are written together are actually pronounced separately.

For example, the French word “Noël” isn’t pronounced like “nole”, but like “no-el”. In German, the two dots diacritic represents an umlaut when written over “a”, “o” or “u”; but over any other letter, it represents a diaeresis. This is very rare, but the text you’re showing us is a translation from Ancient Hebrew, and names of people and places follow different phonetic rules than German does and are often difficult to transliterate.

So, for “Seïr”, you’re being told not to pronounce “ei” together, like in the word “Eis”, but separately: it’s two syllables, “Se-ir”. Similarly, you also have the word “Rafaïter”, pronounced “Rafa-iter”.

used widely in Scandinavian languages Swedish / Norwegian, as far as i know.

What a naïve question! /s