Okay, back, and I think I now understand. So, please allow me a moment to state how I understand it as this was a bit of a jumble as I wrote the first top level.
So, commentary isn't implied that "commentary exists" or that "there is commentary in the commentary box" like would be used in an art tag for a carrot if the drawing has a depiction of a carrot. Instead, what it is being used for is similar to translated tag for translation_request.
Functionally then, it would have any and all commentary that is provided to require one of two tags. commentary_request and commentary. One for is when it is easy for an English to understand and the other for when it is in a different language and requires translation, not unlike translation_request.
The language tag is used for any language that is used for the commentary other than japanese_language, since there doesn't appear to be a tag for it.
If I am following right (and I think I am, but this is for my clarification), what do you have when an art piece has a Japanese title, but no written commentary? As I am understanding it right now, that would be commentary_request and that's it.