Screenwriting : How to format a script in two languages? by Pablo Castrillon

Pablo: who is your reader? Will your reader be bi-lingual? Pick ONE language and stick to it. Then write the Spanish dialogue in English --- adding an asterisk with the note "This is said in Spanish." But if your reader will be Spanish, write the entire script in Spanish. Good luck.

Add your Character and inside the parentheticals write (SUBTITLED - SPANISH) then under dialogue write whatever you want your character say in English or in Spanish if your director or actor knows your Spanish. Writing Subtitled in parentheticals tells the reader that if your screenplay goes into production, a viewer will hear ACTORS speaking in Spanish and see English Subtitles.