A few major languages exists withinthe Empire. We get glimpses of them in the TV-Series, Star Trek XI, FASA-"The Romulans", The "Way of D'Era" and of course Diane Duanes "Rihannsu"-novels.
The most common and now official language is called Rihansa. The Remans (Havrannsu) have another language and scripting system.
Romulan Language Institute for Rihansa (with dictionnary and grammar section) can be found HERE.
Diane Duane about inventing the romulan ('rihansa'-)language:
"...Well, we knew, and know, very little (in the canonical sense) about Vulcan grammar. But what about the sound of the language? If the Romulans were (as I increasingly began to suspect during the writing of Enemy/Ally) an offshoot of the Vulcan culture who might equally have been thrown out, or might have left it on purpose, would they necessarily sound that much like the Vulcans any more? Mightn't it be possible that they had extensively retooled their own language to emphasize the difference between them and their parent stock? (The history of the Pravic language, in Ursula K. LeGuin's The Dispossessed, probably gave me this idea.) And if there's anything we know about the Vulcan language, it's its harshness: full of fricatives and other sharp sounds.
So there at least was a hint. I wanted the Latinate sound, all right, but also intended to suggest that the Romulans / Rihannsu-to-be had purposely gone for a more melodious-sounding language, as a reaction against the hard sounds of Vulcan. So I thought that the language should be strong on liquid combinations and soft diphthongs. Welsh suggested itself to me as an example, right away: so did some of the vocalic ingredients in Greek. I started coining some words to get the feel of it, and produced a few pages' worth.
At this point, I ran across one of my problems with inventing languages. Not grammar -- exotic grammars are easy to build. And if you can't build, you can always steal. Real languages are often much crazier and less logical than anything one would normally invent: look at Irish, or Maltese, for good examples of this. (Indeed, Mark Okrand doesn't appear to have hesitated in this regard: Klingon orthography looks a whole lot, to me, like some of the orthographies used to write various Native American dialects in the early part of this century.) My problem was that, when coining "alien" words, I tend to get in a rut: they start sounding alike. I don't know if other writers have this problem. I just knew I was going to have to find a way around it.
And, lazy naughty creature that I am, I found it. One of the other languages I have a nodding acquaintance with is BASIC. I am not a great programmer -- I have too many other calls on my time to become really elegant at it -- and Heaven knows that BASIC has been bypassed by other languages far more sophisticated. However, it suited my needs. Having decided what kinds of sounds I wanted to appear in Romulan, and how frequently they might appear, and having decided what combinations looked ugly and should be "impossible", I then spent a week or so writing a quick-and-dirty BASIC program which would take numerous consonantal and vocalic "building blocks" I had chosen, and combine them to create words that would avoid my own "ruts" or repetitive tendencies in word-coining. The program would produce several thousand words of Rihannsu in a given run, in a format that looked kind of like blank verse.
Once it was running, I must have gone through about half a ream of paper this way, generating words, and then going through the output and choosing the words I best liked. Ael's name came up in one such run. To that word, and others, I assigned meanings as I wrote the novel. And that was all the work I did on the language, except (when Peter and I sat down [during our honeymoon!] to write The Romulan Way) to collect all the words together into a glossary, which appears at RomWay's end.
Nowadays it seems unusual for a week to go by in which I don't get a request for Romulan language information. I don't have any more, alas, except what you see above, and what's in the books. But I have no problems with passing my tools along to those who feel like using them...." (taken from: http://www.scribd.com/doc/955235)