I actually haven’t started with this at all, but just to note it down, the RSS feed of pangya.jp was also archived for a long while.
https://web.archive.org/web/20170511211914/https://www.pangya.jp/rss/rss.rdf
It might not be enough for a full log of patch notes like with Global, but there still might be a few versions we could get details for.
DaveDevils also helped with a script here!
A lot of the patch notes of Japan just included links to news/forum pages, which sadly have not been archived properly, and other parts have very complex HTML markup in them which the wiki can’t handle that well, but for the most part it still looks somewhat fine.
The version history we have for Japan is reaching from 2005 to 2017!
https://pangya.wiki/wiki/Category:Japan_Patch_Notes