inakoooo: ... I've tried, but it keeps working in English. I don't know if it will important, but when i installed the game, i could not launch the game because it lacks OpenAL32.dll, then i installed the .exe file of this post:
I don't think it's connected with OpenAL.
The configuration of the game is stored in \My Games\HotlineMiami in your My Documents folder.
Go there and open hotline.cfg with any text editor.
Edit the language entry to 3, because that's Spanish. Like this:
language=3
Save, close and run the game.
If you run the game with another account (e.g. an Administrator account) the game will create another hotline.cfg file in this users "My Documents", hence these settings only apply while running the game with this account.
EDIT: Apparently there was something wrong with the Windows user profile, so that the stored configuration wasn't loaded correctly; Creating a new user, re-downloading the game and installing it with this new account solved the language issue.