3) The transmission was garbled, and the Romulans said Romulan but Hoshi didn't quite catch it, and the famed Universal Translator was of no help in spell-checking alien proper names.
Number 3. That was her first run-through of the language, and Hoshi might've only heard the species name once amongst a tough-to-understand translation. It's "something called the Romalin Star Empire," not "the Romalin Star Empire," so Hoshi might not be so sure if she picked up the name.
T'Pol, having heard or read of the Romulans, corrected Hoshi's pronunciation.
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As for the language-issue: There must be two main Romulan languages, which are completely unrelated to each other. The one that Hoshi encountered (and probably used throughout the Romulan War and perhaps when dealing with outside races in general), and the one that was indistinguishable from Vulcan (probably the one used on Romulus amongst themselves).