Especially when it comes to transliterating alien words into English which have never been encountered before.
But Hoshi would not have "translated " either Romalin or Romulans into English,
I said "transliterate," not "translate."
it's a proper noun, in this case obviously a name. Hoshi would have simply repeated exactly what she heard. She would have then translate the follow on "Star Empire" into English. But not the name itself.
You're making several unsupported assumptions about the nature of the Romulan language. You're assuming that all of its phonemes can be replicated into English, you're assuming that its adjectives are used as separate words, and you're assuming that it uses the same syntax as English.
Maybe Romulan is like Spanish, and the adjectives succeed the word being modified (except when they're trying to be archaic and fancy, at which point they may precede the adjective) -- "Empire Star Romulan" rather than "Romulan Star Empire." Maybe the phonemes used in Romulan can't be completely accurately replicated in English, and both "Romalin" and "Romulan" are accurate transliterations (and T'Pol was just being an ass, as per usual).
Or maybe the adjective that corresponds to "Romulan" isn't actually a separate word, but is, rather, a prefix or suffix added to their word for "Empire." In point of fact, perhaps it's a prefix or suffice whose pronunciation
changes upon being affixed to another word -- so that it's pronounced "Romulan" in most circumstances, but "Romalin" if used as an adjective. (For a comparison: The term "biopic" is a word that has recently evolved, combining the words "biographical" and "picture," to describe a movie about a person's life. Yet when it is spoken out loud, it is often pronounced as "bi-ahp-ic" rather than "bio-pic.")
Languages are often full of funny things like that. Maybe an accurate replication of the phrase corresponding to "Romulan Star Empire" would be literally something like "EmpireRomalin-of-stars," with "Romulan Star Empire" being a way to make the phrase make sense with both a consistent pronunciation of the culture's name (keeping it "Romulan" at all times rather than just when used as a noun), by separating the adjectives from the noun, and by removing the "of stars" clause and changing it into a mere adjective ("Star").
Changing the literal meaning when translating something is also common. In Spanish, for instance, you wouldn't say, "I am 25 years old," you would say, "Tengo 25 años," which literally translates as, "I have 25 years." Similarly, in Spanish, you wouldn't say, "I like Mister Spock" ("Disfruto del señor Spock"), you would say, "Me gusta el señor Spock," which would literally translate as something like "The Mister Spock is pleasing to me" -- or, more literally, if you preserve Spanish syntax, "To me, is pleasing, the Mister Spock."
Bottom line: There could be plenty of reasons Hoshi thought it was "Romalin" rather than "Romulan," ranging from honest disagreement to how to transliterate the term into English, to genuine differences in how the term is pronounced in Romulan between its noun and adjective forms, to the fact that, yes, even geniuses sometimes make simple mistakes and that's just life. It's pure thoughtlessness to just assume it's bad writing.
Haven't we already been warned about passive-aggressive smiley faces?
ETA:
Seems too basic a mistake to make for someone like Hoshi.
Pop quiz! Is "北京" pronounced as "Peking" or "Beijing?"
Have you ever studied how words from different languages are transliterated? How there are literally conflicting systems for how to transliterate words from one language to another?
Even a trained linguist can make a mistake. Especially when it comes to transliterating alien words into English which have never been encountered before.
Usually that happens when there is a third party involved rather than a direct translation. Its why "Germany" and "Japan" appear on English Maps.
You mean sort of like how T'Pol, in "correcting" Hoshi's pronunciation, was a representative of the planet of Vulcan and was therefore a third party to that particular Human/Romulan incident?