I use Japanese on a daily basis, but my computer is set to English. Whenever I view a page in Japanese, asked translate it. It would be useful to be able to select a language to be translate, or to remove language from the translation service.
You should be able to specify that you never want to translate Japanese in the Full-Page Translations Panel Settings (gear-icon menu) when visiting a website in Japanese.
We also have an upcoming project that should make it easier for users to manage these per-language preferences in the main Firefox Settings.
(This is a work-in-progress screenshot and may not be represent the final version.)