Maybe to challenge Deckard. Deckard refuses to play by Gaff's rules (street rules) then, but by the end of the film Gaff does speak English with him ("Too bad she won't live!").
In Blade Runner 2049, I don't recall cityspeak being used at all, and the scene with Gaff being interviewed by the new Blade Runner is entirely in English.
In Blade Runner 2049, I don't recall cityspeak being used at all, and the scene with Gaff being interviewed by the new Blade Runner is entirely in English.