I am not sure if I can explain it well since I am using DeepL Translate. 
Simply put, the current Japanese file for Grim League requires language.def with Wordmode=false because the character folding process is incomplete with only Text_ja.arc.
You can PM me in japanese on discord. 日本語すこしできるよ。お前は日本語のほ翻訳したいですか？おれはファイルを提供できます。
Not sure where to post this but there is currently a bug in grimtools. On legendary sets, where there is a granted skill, there are currently no details describing the skill. If you look at the same set under Grim League season 7 it is there, but not in vanilla. Try it out. Look at the Wildblood set or the Deathmark set on vanilla then on Season 7. Tell me what you notice.