Okay, I'll try a few methods on my own for a while.
About translating custom mod translations, it didn't completely fail, I was able to translate "RecipeBrowserUI.ModFilter" and "CraftUI.NoPathsFound" as shown in the image below.
By the way, "RecipeBrowserUI.ModFilter" was translated after I...
Hmmm... from the looks of it, this mod uses lang files to manage the translations. I have a Japanese translation of en-US.lang, but I don't have the skills to replace the lang file from another mod. What should I do?