I imagine if youre playing in japanese they'll just give people accents, similarly if you're playing with filthy dubs they'll just give the japanese an accent...except for kiryu who'll sound like a lifeless shell of a man
From what I played in the demo, the enemy taunts and some other random npc's will say english words but everything else story and side story wise will be fully japanese. Im sure they'll prolly make a joke about Ichi imagining everyone speaking Japanese but in fact they all talk in english. Dont worry too much about it. Dont play it dubbed.