You can download the Spanish (or any other language) ASCII dictionary from http://www.winedt.org/dictASCII.html and then add it to Android Studio as sebaszw said:
- Go to Settings -> Spelling -> Dictionaries
- Click plus symbol(+)
- Select path to your dictionaries folder (inside you must have plaintext word lists with .dic extension)
- Restart Android Studio
It worked for me without restarting Android Studio