Screenwriting : In Spanish by Reynaldo Leal

I use parentheses for the first instance with the spoken language indicated - EG: (in Spanish), and write the text in italic. All future instances of text written in italic will be spoken in that language until stated otherwise - EG: (in Italian).

I second Stefano - although if you want the opposing character to not understand or something along thoe lines, feel free to type it in Spanish as it easily communicates that.

Welcome, Reynaldo Leal. If a character speaks Spanish, I write it in English, then put "In Spanish" in ( ). If a character (or two characters) speaks in Spanish for a long time, I put a note in action description (something like "She speaks in Spanish") and bold the note. I do this to save space.

As long as you write it in a way where your intentions on how the audience will see it are clear its ok. The easiest way to do this is to write the English dialogue normally and under the character heading write for example "(In Spanish, Subtitled)" for the Spanish dialogue and write that dialogue in English but in italics.