@lvxferre still, with a whole-ass 34 years of using only Japanese (and maybe some english), if he
was ported over with soul and a full memory imprint from his previous world, then I'd wager that it's one of these to things:[ul]
* not quite the classic memory effect where you get out of practice; he may by virtue of the "full imprint" be "cursed", if you will, with perfect recollection of his former life
* possibly just invented a memory spell granting him perfect recollection of his former life (the memory imprint, if you will)
[/ul]
My point being, there's a bunch of in-universe explainations that could be theorycrafted to fix that plot hole.