I always sort of assumed "dialect" basically meant what we would understand as "language." I mean, English and Japanese are likely to be far more similar to each other than English or Japanese is to Klingon Dialect No. 46. (They may even be real cousins, albeit distant, if human language has an unbroken lineage. The Japanese people
are real cousins to Anglo-Saxons, after all.)
I've also supposed that some or many alien species have constructed languages, ala Esperanto, specifically designed for realtime translation and thus interaction both between different ethnic groups and between other species. I'm no linguist, so I don't know if some languages might be more amenable to realtime translation or not; I used to think so, but now I'm not sure.
Romulan, being the product of a relatively small and perhaps ethnically homogenous founder population that definitely had recording technology, really could be just
one language. Like say if there was a one-world government coming and ten million Americans said "F it" and took off for Alpha Centauri, we could reasonably expect them to speak English, even when there were the far-flung billions of them two thousand years later.