In Star Trek we occasionally hear alien languages being spoken, notably Klingon every now and again, and Vulcan features infrequently. The third major alien species that features regularly is the Romulans; I don't recall hearing it, but do we ever hear their language being spoken?
There was an episode of Enterprise where they first encounter Romulans and before the ship's computer can render a translation, you only hear Romulan. This is what is heard (from Memory Alpha):
Uhn kan'aganna! Tehca zuhn ruga'noktan! Uhn Kan'aganna! Tehca zuhn! Neemasta kan'aganna uckwazta! Kuhn'ukchtacht zuhn vockwadai! Bar'ak t'stu annankana.
The Memory Alpha page provides a small list of phrases and words that we hear or see in the franchise:
- D'deridex: "Warbird", a term designating Romulan starships, or perhaps a particular class of starship (heard several times in TNG)
- Jolan Tru: A salutation, used in the same way as "greetings" and "good-bye". (TNG: "Unification I", ENT: "United")
- Prod: Praetor or Commander. (Star Trek 2009)
- Uhlan: A rank in the Romulan military. (TNG: "The Defector")
- Veruul: A churlish or rude person; possibly profane (TNG: "The Defector")
- Viinerine: A type of food. (TNG: "Face of the Enemy")
- Vorta Vor: The source of all creation. (Star Trek V: The Final Frontier)
In terms of seeing the script, we see the following script on Geordi's VISOR in TNG: The Mind's Eye:
There is also quite a far more extensive understanding of the language provided on Memory Beta, but these are outside the scope of the question (which refers to language heard on-screen)