there is no letter J in the hebrew language.
There is no "Y" either--Hebrew doesn't use the Latin alphabet. J and Y, as well as the other 24 letters in the English alphabet, are just symbols used to show what sound is used in a word. It's correct that in English, God's name is "Yahweh". It's spelled like this because the letter "Y" is the sound that we use in English to symbolize the "juh" sound. In many other languages, like German, God's name isn't Yahweh, but Jahweh, because "J" is the letter used to symbolize the "juh" sound.


