The name John derived from the Hebrew Yohanan, which is Ioannes in Greek and Johannes in Latin (pronounced like the Greeks did). In this ATL Christianity doesn't spread that far outside of a small Jewish sect. As a result any of the transliterations of "John" don't really gain popularity. As the Jews spread out in the Diaspora Yohanan (which was the 5th most popular Hebrew name at the time) travels with them. One of the places they settle is Arabia, where the name is transliterated as Yahya. Eventually secular Arab leaders unite the Peninsula and go out to conquer much of the Levant. Slowly the name Yahya gains popularity amongst the Arabs (many of whom convert to a form of Judaism that doesn't exist IOTL). In Europe trade with the Levant eventually transliterates the name as Jahya (Germanic), and Jahanyas in Latin. (the Roman Empire still exists, but is falling, by the time of the Arab conquests) Local dialects drop the "yas" and so the most common form of the name is some version of Jahan.