[[ It’s not in a noble, stuck-up way. But it’s not entirely too casual, too. It’s in the middle, there. Itachi has some vocabulary that is not normal for people his age, given the whole ~more mature than others~ thing he has going on. He is more intelligent, and rational in a lot of cases, and his speech reflects this. He doesn’t show too much emotion in his speech and in his Akatsuki time it’s really close to levels of monotone. (to keep the cold, emotionless killer mask up.)
But he’s also not reading a thesaurus at you, or refusing to use contractions ala Lee. To be fair on the part of English, they have a hard job ahead of themselves, to match things like character speech styles vs mouthflaps. It’s why Naruto’s catchprase used to be Believe it, before it was “Yanno/Y’know”.
Personally, I’d like to rewatch the English dub to nail his speech mannerisms down a bit more. The voice I have is through years of practice but may be rusty. …Plus I miss Crispin Freeman. ]]