If I have a character who speaks in English but curses for instance in spanish, do I write the dialouge in spanish itself or put (in Spanish) next to the dialogue?

For example

                                            CASSANDRA
                                Its all we can do! Now get them to the
                                safe room before they all end up dead!
                                Dios now asista.

Or
                                            CASSANDRA
                                Its all we can do! Now get them to the
                                safe room before they all end up dead!
                                (in Spanish) God help us.
                                        

Example #2

I personally write this at the start of the script: "NOTE: ALL ITALICISED DIALOGUE IS TO BE SPOKEN IN _______ (INSERT THE SPECIFIC FOREIGN LANGUAGE), AND SUBTITLED IN ENGLISH."

I feel it takes up less space than using parentheticals, especially if the character will be switching between languages a lot.

Maybe if you're doing it just once or twice, use Example #2. If it's going to be a lengthy affair, consider Toby's example.

What Toby said.  

Make a note of it before a character starts speaking.  I do this in plenty of my scripts, even with accents (southern, redneck, English, Scottish, French, Spanish), mainly because my translation module sucks!  Then it's up to the filmmaker to get it right, HAHA!

So it'd be:

NOTE - Cassandra is Spanish, all italicised dialogue to be spoken in Spanish.

                            CASSANDRA
                 It's all we can do! Now get them to the
                 safe room before they all end up dead!
                 God help us.

Of course, if you have a character that ONLY speaks a certain language, this does not change much, but you should add this:

NOTE - Martinez is Mexican, all of his dialogue is in Spanish with English Subtitles.

(in Spanish) Thank you.

Both work. If you aren't sure if the Spanish translation, just use parenthetical. I think if you can do the language, consider doing it...but...if there's any doubt about the reader understanding, you have to do in English.

I was recently reading a Cormac McCarthy novel. He slips into Spanish fairly frequently. At first I didn't mind because my Spanish was just good enough that I could figure it out. But then he did a "scene" that was crucial to the story, and he did it in so much Spanish that I couldn't figure it out. This caused me to put the book down.  

Quoted from CameronD (in Spanish) Thank you.

eres bienvenido

Quoted from oJOHNNYoNUTSo Maybe if you're doing it just once or twice, use Example #2. If it's going to be a lengthy affair, consider Toby's example.

I definitely agree with this. In the script that I am currently working on, there are characters who for the most part, speak only in foreign languages. As a result, it was easier to use italics versus parentheticals. If it was going to be an isolated incident, I would just take the parenthetical approach, though.

I wrote a script in which a character spoke both in English and Spanish and I simply wrote out exactly what she said (most was English), without any wrylies.

If the character only slips into Spanish to curse, I don't see a need to break up the dialogue with a parenthetical.  Who wants to read this:

MARIA
I can't believe you cheated on me!
(IN SPANISH)
Son of a bitch!
(IN ENGLISH)
Pack your things and leave --
(IN SPANISH)
Asshole!

When you can just write this:

MARIA
I can't believe you cheated on me!  
Hijo de puta! Pack your things and
leave, cabron!

I wouldn't even bother italicizing.  It's a couple words, not an entire stretch of dialogue or even one sentence.

I agree with Lon.... and Jeff.