I have a question regarding languages

I have yet to fully shift so I don't have any experience on this, but why don't I ever see anyone claiming to know a language they never did after coming back from their DR. like maybe if someone were to shift to an anime DR where the language spoken is Japanese, why don't they seem to understand Japanese after being in their DR for a while cuz even if you've never understood that language before when you shift you automatically do cuz your DR self does right? so why doesn't that knowledge/ability stay? i understand not being able to speak right away cuz your CR-self mouth is not used to its pronunciations and stuff but wouldn't you be able to still understand it cuz you were conscious of it? I hope this made sense, English is not my first language.