"The most obvious proof that Jehovah's Witnesses spell YHWH wrong as "Jehovah" is the fact that the letter "J" doesn't even exist in Hebrew, Greek, Latin."
I thought this was pretty interesting.
Most Hebrew Scholars translate YHWH as Yahweh, a verb form meaning "to be" which is backed up in scripture in Exodus where God reveals his name to Moses as "I AM THAT I AM". How do some scholars come up with any other interpretation, such as YHVH, JHVH and JHWH? Is there any Biblical support for these translations or are they just mistranslated?
Shiloh covered the error of the wrong vowels in Jehovah.
Now for the others. The third letter in the Tetragrammaton is the Hebrew letter 'vav'. It quite often has the sound 'v'. However, there was a time of transition, when the letter used to have the 'w' sound. So, some will say Yahweh, and some will say Yahveh. You will find in modern Judaism, which would not pronounce the Tetragrammaton, if they did, the 'v' sound would be used.
Now for JHVH. Hebrew does not have the letter J, or really the letter Y either. The name for the Hebrew letter which has the y sound, is yod. Hebrew has a different lettering, so neither Y or J exist in Hebrew. Hebrew does have yod, which has a sound similar to our modern Y, and Hebrew does not have any letter with a sound similar to the modern J. But, going back in history, with the Roman alphabet we use today, in Jesus time, there was no letter J in the alphabet. The letter J was added later, and when it was first added to the alphabet, it actually had the Y sound. I think Spanish, has the J which sound like a Y. So, J was used quite often for the y sound. But later, the J changed sounds and is no longer ever the sound of the Y. I think that is why people picked up the J from older writings, when the J had the Y sound, so Hebrew letters Yod, were transliterated to a J.