I have never in FFXIV seen the words JP only or JP onry, but most JP shouts I see are entirely Japanese, I might try to jump in one sometime see what happens, but I do agree that it has alot to do with the language barrier, for example I was in an ifrit fiight with 2 Japanese people, we had somone in the party that had not tanked it before so the Japanese people were telling them tanking positions (how JP do it) now the tank did know japanese, but so we all knew what we were doing they had to communicate in english and it was quite confusing because they didnt speak much english.

Personnaly I respected them for communicating in english when they didnt know much, but I think some of the confusion over communication could have been one reason why we failed and didnt fully understand how we were doing it, language barrier can be a rough thing to get over.