The Spanish correct form of the name is Jonatán (khoh-nah-TAHN), but, as many other names from Old Testament, it is not used.
The use of
Jonathan appears in 1970s after American films, soap operas and sitcoms; then, the pronunciation tries to be the English one. Since the sound of the English J is inexistent in Spanish, many of the speakers pronounce it as Y (it is the same with other newly used names, as
Joel). The group TH is pronounced simply as a T.
Lumia
http://onomastica.mailcatala.com