(covers information from several alternate timelines)
The Romulan language was used within the Romulan Star Empire and the Romulan Free State. Its written form consists of square and rectangular letters, which could be arranged horizontally or vertically. (e.g., TNG: "The Mind's Eye", "Face of the Enemy"; DS9: "Inter Arma Enim Silent Leges"; Star Trek Nemesis)
In 2370, Jean-Luc Picard identified an artifact recovered from Calder II as being from Vulcan, because the alphabet and symbology of the glyphs and pictograms on the artifact was much more consistent with early Vulcan than Romulan. (TNG: "Gambit, Part II")
Upon the first official encounter between Romulans and Starfleet in 2152, audio-only communications were established. Although the early universal translator initially had problems to lock on to the Romulan language, Enterprise communications officer Hoshi Sato was able to translate what turned out to be an ultimatum to depart immediately. (ENT: "Minefield")
The Artifact was a joint project between the Romulan Reclamation Site and the Borg Reclamation Project, thus announcements, signage and crew conversations tended to be in both English and Romulan. (Star Trek: Picard)
Words and phrases
- Chula – Place name of a valley on Romulus (TNG: "The Defector")
- D'deridex – "Warbird", a term designating Romulan starships:
- Gal Gath'thong – A place name on Romulus (TNG: "The Defector")
- Ganmadan – Ancient term for the "Day of Annihilation" (PIC: "Absolute Candor")
- Jolan tru – A salutation used for "hello" and "goodbye" (TNG: "Unification I"; ENT: "United"; PIC: "Absolute Candor")
- Kali-fal – A type of Romulan ale (DS9: "In the Pale Moonlight")
- Lu shiar – Lifting the eyes; second segment of Zhal Makh (PIC: "The Impossible Box")
- Pixmit – Cards with cultural significance (PIC: "The End is the Beginning")
- Prod – Praetor or commander (Star Trek)
- Qalankhkai – "Freeblade", a Qowat Milat warrior bound to a worthy cause (PIC: "Absolute Candor")
- Qazh – a profanity (PIC: "Nepenthe")
- Qezh – a profanity (PIC: "Broken Pieces")
- Qezhtihn – An insult (PIC: "The End is the Beginning")
- Qlam wath – The Unfolding; third segment of Zhal Mahk (PIC: "The Impossible Box")
- Qowat Milat – An order of warrior nuns (PIC: "Absolute Candor")
- Rok khan – A later segment of Zhal Makh (PIC: "The Impossible Box")
- Shaipouin – "False door" (PIC: "The End is the Beginning")
- Tal Shiar – The Romulan intelligence agency/secret police (TNG: "Unification I")
- Tan qalanq – A sword wielded by the Qowat Milat (PIC: "Absolute Candor")
- Uhlan – A rank in the Romulan military (TNG: "The Defector")
- Veruul – A Romulan insult, analogous to the Klingon word "pahtak" (petaQ); possibly profane
- Viinerine – A type of food (TNG: "Face of the Enemy")
- Vorta Vor – The source of all creation (Star Trek V: The Final Frontier)
- Vri glam – The center; the fourth segment of Zhal Makh (PIC: "The Impossible Box")
- Yut makh – Closing (as of the eyes); starting point of a Zhal Makh (PIC: "The Impossible Box")
- Zhat Vash – A term sometimes used for the dead (PIC: "Maps and Legends")
Examples of spoken language
- Setha-tri par trukatha.
- Setha-ki par trukatha.
- Setha-mille par trukatha.
- Part of a Romulan self-destruct countdown aboard a Romulan Warbird. (TNG: "Contagion")
- 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.
- Romulan ultimatum to Enterprise NX-01 inside an annexed star system. (ENT: "Minefield")
- Yiwav sooha wassyakassa kahfavret eedayhoon fal teh voh?
- If certain people requested entry... would it be granted?
- Nehn vrelev zohzuus.
- We are not like them anymore.
- Lehsbey invehcreese nehveltev staam.
- My presence is an intrusion, forgive it. (PIC: "The End is the Beginning")
- Shauwdaur botwar!
- Run, sisterboy!
- Sharah aroostos!
- Bite me!
- He's coming!
- La-hok yuun uzor nek varet viyuun.
- Let yourself be seen.
- Ramdha shabakom, widuus fidufi wass kai Ganmadan.
- Professor Ramdha, what can you tell us about Ganmadan?
- Ganmadan n'rihaar falakfar dranbst steb faifsneh mahktaar.
- Ganmadan is what our ancestors called the Day of Annihilation.
- Sa viduus dor kehm 'mahktaar'...
- When you say 'annihilation'...
- Daneek pal. Dal gevuul, nigau. Zha hass velavoor zekha trekt logevlaza ektoi sorat Seb-Cheneb.
- Of everything. All life, everywhere. When all the shackled demons break their chains, and answer the call of The Destroyer. (PIC: "Absolute Candor")
- Nimm vrakesh.
- Not for her. (In response to "Is this room available?")
- Zhal Makh nimm vramin gadar.
- Zhal Makh is not for round-ears.
- Ikell uhklet nem vrann ilell. Vijot uhl chaltov.
- And a sub-commander's bars are not for idiots. You risk losing them. (PIC: "The Impossible Box")
- Vesh ta'jot.
- Kill them all.
- Feldor staam toreht.
- Please, friends. Choose to live.
- Rashant... sudan ropengneh nah.
- I admit... I was not expecting this.
- Nenhahn impahr zhatu Zhat Vash val Qowat Milat konin.
- This is not how Zhat Vash fights Qowat Milat.
- Shet nigosh b'bat. Gorosh.
- A crewman aboard the Narada, after Captain Richard Robau comes aboard. (Star Trek)
Romulan dialogue was spoken by Ayel (played by Clifton Collins, Jr.) in a deleted scene from the film Star Trek. Translated into English, the Romulan he said meant, "Commander Nero. We're surrounded. What are your orders?" The Romulan dialogue written for the show was supplied by Marc Okrand, better known for creating Klingonese. "I had never done anything with Romulan before, but since Romulans and Vulcans are related, I made the Romulan language one that could be related to Vulcan – not closely, but in certain patterned ways. The two languages are heard mostly in the background as opposed to being spoken by main characters." 
For Star Trek: Picard, constructed language creator Trent Pehrson was hired to develop the grammar and vocabulary of Romulan language spoken in the series. Pehrson also served as a dialect coach for the language.  "I was given instruction on what preexisting fragments were to be considered as canon. I incorporated all of those. Essentially, those really only yielded limited phonotactic information, some vague lexical items, and a hand-full of possible grammatical morphemes. There was also some Vulcan canon, which was useful, in a historical linguistic sense, to further flesh out phonotactics, and to derive another small set of lexical items. Native Romulan orthography fragments, used in prior canon production, were aesthetically pleasing, but were clearly just a thinly disguised version of the Roman alphabet. So, I used only the visual aesthetic from that, and created a system fitting to the actual phonotactics and phonemic inventory of the Romulan language. ST:Picard, E2 recently featured a decent sampling in a scene. The rest (the majority of the language) I had to construct." 
- In Diane Duane's novel The Romulan Way, the Romulan language is said to be related to the Vulcan language. This suggests that during the Exodus of the Romulans, they abandoned most things Vulcan. "One thing they agreed on quickly was that they could not stop being Vulcan while they still spoke the language." In order to rectify this problem, the Romulans went back to old or "High" Vulcan and "aged" the language in another direction. The resulting tongue was named Rihan, with the Romulans' name for themselves being Rihannsu in Duane's novels, meaning "the Declared", in reference to their decision to secede from Vulcan society. (This name also used to collectively refer to the novel series.) The Romulan Way includes a partial glossary for the language (reprinted in the omnibus edition Rihannsu: The Bloodwing Voyages), and Duane reportedly intended to publish a Rihan counterpart to The Klingon Dictionary but it never materialized. However, fandom has extensively expanded on it, and it has been reused in other licensed works including the Romulan War series of Star Trek: Enterprise novels and Star Trek Online.
- In the novel Vulcan's Heart, Spock notices Romulans cursing in old Vulcan.
- The Romulan language font is called Kzhad, and was created by Monte Thrasher based on the shapes of LED displays.
- FASA gaming manuals referred to the Romulan word for the Romulan language as "Romlastha".