I’ve just been getting a whole bunch of Romulan headcanons lately, I don’t know why. Here’s another one:
Vulcans probably have hundreds of languages and dialects spoken, since they’re inhabiting the planet where they evolved for bajillions of years. However, since the Romulans seemed to be a somewhat unified group (perhaps a single nation or single ethnic group of Vulcans, or perhaps a cross-section of Vulcans unified in their rejection of theocratic rule), I think it is likely Romulans speak only a single language, or a handful at most.
Since the Romulans left in an era of modern-ish technology (they had space travel), I’m assuming that they had also invented audio recordings and writing by that point, which would have slowed linguistic drift. Thus, I think Romulans likely speak a language that is intelligible on Vulcan, despite the Sundering being ~2,000 years ago. I believe the Romulan dialect sounds particularly sing-song and lilting, even to human ears but especially to the ears of Vulcans who speak in a very deliberate monotone. I mean, all languages probably sound lilting to Vulcans, but I think Romulan does especially, since they’re speaking familiar words but in a strange sing-song manner.
The lilting accent is the first thing undercover Romulan agents and spies have to lose if they’re assigned to Vulcan. Of course they also have to learn Vulcan’s conventions and face culture. I think Vulcans, socially, would have endless Byzantine rules, unwritten expectations, and social conventions that you must follow perfectly, else you commit social faux pas and are considered weird or even become ostracized.