It was the wording of the question honestly that I think made it difficult to answer in any way other than "lol i duno". Asking specifically if Doman/Hingan is flat out Japanese allows that "well in Japan...no" loophole. The way I'm seeing it is that it's flat out a different language, but it's not straight Japanese just like how the old Elezen tongue isn't straight up French.
For further reference look to earlier in the panel and he talks about how the xaela tribes all have their own tongues and but they also have another language in common so that they can properly communicate. They make it clear that a lot of the words in said language are derived directly from Mongolian but there are other words just just created to make the language its own distinct and separate thing. Essentially they don't seem to want to directly grab any one language and use it verbatim as a the same language with a different name.
While it is very much "do as you like" it seems to come across that there are definitely outright language barriers and the game is presented as is for the sake of not having to go and have everyone struggle to understand eachother constantly.
EDIT: Essentially what I'm saying is...if you want to RP a different language, just use brackets. There are clearly lore supported words/terms/phrases and you can use those normally, but there's really no need to have your character flat out speak Japanese and say it's Hingan...just use brackets and say it's Hingan.
For further reference look to earlier in the panel and he talks about how the xaela tribes all have their own tongues and but they also have another language in common so that they can properly communicate. They make it clear that a lot of the words in said language are derived directly from Mongolian but there are other words just just created to make the language its own distinct and separate thing. Essentially they don't seem to want to directly grab any one language and use it verbatim as a the same language with a different name.
While it is very much "do as you like" it seems to come across that there are definitely outright language barriers and the game is presented as is for the sake of not having to go and have everyone struggle to understand eachother constantly.
EDIT: Essentially what I'm saying is...if you want to RP a different language, just use brackets. There are clearly lore supported words/terms/phrases and you can use those normally, but there's really no need to have your character flat out speak Japanese and say it's Hingan...just use brackets and say it's Hingan.