So he was able to speak & understand immediately instead of needing to learn the language as he grew. So this kingdom's primary language is also Japanese?
I think the pacing is too slow. I know the first chapter is supposed to be something that hooks the reader in, but there is something wrong when it takes 3 whole chapters that are 40+ pages long just to start the premise.
Not gonna lie, for a moment, I thought this was Million Arthur related. Index's artist, Excalibut, someone being king Arthur, carelessly handling excalibur, the synopsis sounding like the insane stuff that Kazuma Kamachi always writes, etc.