Hi Corvin, thanks for your question. The standard pronunciation for J is the /dʒ/ sound as in gentle and just. The pronunciation as /j/ after a consonant is not canon and should be disregarded.

Here's an example in Paarthurnax's dialogue with the pronunciation /dʒ/. I can't recall any instances of /j/ in Paarthurnax's dialogue, but I'd be grateful if you could point them out. (The linked video doesn't contain the entirety of his dialogue, so you may have to go digging elsewhere.)

If the word that caught your attention happens to be "hofkahsejun", it is also good to remember that Paarthurnax pronounces "suleyk" as /'sulʲex/ once. Though the voice actors were instructed quite well, mistakes happen.

Okay, so I returned to a previous save to hear what it was that Paarthurnax said and I found a few different things. whenever Paarthurnax or Alduin say "Joorre" they pronounce it as "Joor" though for other plurals they change the pronunciation. Alduin also says "Sillesejoor" with a /j/, though he does not pronounce the e between the s and j so that makes some sense. The same happens when Paarth says "Hofkahsejun" meaning dragonsreach. He also says "Bronjun" meaning jarl with a /j/ sound, so how is that rule not canon? I do seem to have been mistaken though, it seems that the only times when the /j/ sound are used is when following a consonant or the silent vowels in compound words.

Ammendment 1:
I was on the memrise canon course and I finally reached the compound words section. The given pronunciation of bronjun was with a /dʒ/ sound, even though Paarth says it with a /j/. Here is Paarth saying it with a /j/ https://youtu.be/-1Oe7ofllL0?t=12m29s
And here is Paarth saying hofkasejun with a /j/ https://youtu.be/-1Oe7ofllL0?t=11m18s