But the Vulcans and Romulans fought a war against each other once. (before either race made contact with Earth)
Still, the Romulan and Vulcan language would be different enough. Look at human language in the 4th century compared to the 21st. Now, add in a lack of communication between the two races for millennia, a total change in culture and teaching, and a people bent on being as secretive and paranoid as possible.
That said, T'Pol may have had
suspicions, but why would she share those unverified suspicions with her human shipmates? Vulcans are secretive as well, though out of what they feel is necessity instead of paranoia. There would be no reason that, if she even knew for certain it was Romulan language, that she could say anything to Archer. It would be some time before she started really pushing back against the Vulcan High Command.