I have the exact same issue; the game is in Spanish and the only languages I can select are Spanish and Korean. Reinstalling Diablo IV, Battle.net, logging in and out, clearing all the settings I can find, setting the TextLocale and SoundLocal value in Documents/Diablo IV/LocalPrefs.txt to “2”, updating Windows, they all do nothing.
Interestingly, clicking “Reset to defaults” / “Restablecer valores” (middle red button at the bottom) in the Accessibility settings in-game causes all the strings in the game to become blank/missing.
Guess I’m learning Spanish now. 
Jokes aside, paying 90 USD for the Deluxe Edition and having this sort of experience feels kinda bad (particularly when the only reason I bought the Deluxe Edition was for early access that I now can’t make use of).