A good suggestion for this is The Lord of The Rings. It has constant use of elvish dialogue, which is sometimes subtitled and sometimes not.
There are two main ways to do it depending on a) how much you want or need to detail in the script and b) whether you actually know the language you need written.
Version 1, dialogue included (from
Fellowship of the Ring (page 153, if you are interested):
Elvish; subtitled)
Am meleth dîn. I ant e-guil Arwen
Undómiel ... pelitha.
For her love, I fear the grace of
Arwen Evenstar ... will diminish.
ARAGORN
(in Elvish; subtitled)
AnÃron i e broniatha ar periatham
amar hen. AnÃron e ciratha a
Valannor.
I would have her leave these shores,
and be with her people. I would have
her take the ship to Valinor.
The other scripts are at:
http://www.fempiror.com/otherscripts/LordoftheRings2-TTT.pdfand
http://www.fempiror.com/otherscripts/LordoftheRings3-ROTK.pdfHere's another way to do it just to hit the subtitled translation more specifically. Arguably, it's kind of obvious, but I figured I'd include it. Yes, this is a more conceited example since it's from The Fempiror Chronicles.
ILDRITCH
(low, in Felletterusk;
subtitled)
Ti sil atef esprültärsh runvajav,
kaki anlandi. Erim silnil hav ti
sik.
(translation)
It is a hopeless venture, my
friend. Erim is not what it was.
ZECHARIAH
(in Felletterusk;
subtitled)
Ka stüra tidä tod. Lew stüra
Tepishel pesti anjo yeh crira
vertes.
(translation)
I have to do it. The Tepish must be
exposed before they are allowed to
take over.
Variation 2, actual dialogue not included:
I honestly can't think of an example for this off hand, but I know they're numerous because it takes less work. Goes like this:
BOB
(in Russian, subtitled)
I don't know a word of Russian, yet I can somehow speak it fluently. Why is this, Korokov?
KOROKOV
(in Russian, subtitled)
I believe the writer knew a guy to translate your dialogue for you.
That's all there is to it. Say a language and write it in English. Not too complicated. I used LOTR for the first example because, being a fictional language, it HAD to be written out.
Hope that covers it.