Not necessarily. How many hundreds, possibly thousands, of spoken languages have there been on Earth alone throughout history - many of which totally unrelated to one-another? Just ask the Japanese cryptographers who tried to track Navajo code-talkers in WWII. They just couldn't do it. How many languages may have died out in the past 2000 years? Romulans may have been a part of the Vulcan species at one time, but there is nothing to say that they share a common language. They might have been a part of a Vulcan nation that split before a unified global language took root, post-Surak. The fonts used on-screen throughout all of Star Trek (Vulcan and Romulan) show two visually different alphabet styles (even multiple versions of Vulcan). It could stand to reason that they could be completely different-sounding as well.Heck, it would seem linguists would be able to begin making the connections that, even after 2,000 years, there are similarities to the languages each speak.