For some reason I don't think that the original Japanese line was "When in Rome, do as the Romans do," nor do I imagine that Haru has the proper frame of reference to understand that particular saying.
Obviously, the original Japanese didn't literally translate into this idiom. A much more literal translation would be "In order to respect the other party's culture, one must actively make an effort to blend in." However, translating this into the well-known idiom "When in Rome, do as the Romans do" was a stylistic choice that captured this exact same message in a more succinct way that is also easier to typeset. When translating, I am an advocate of flexibility (within reason) within authorial intent. While adhering strictly to literal meanings is a choice, it's one that I often find adds additional hardship when trying to structure sentences in English. JP to EN is never a 1-to-1 translation, especially since both languages have widely different grammatical structures and rules to them. You almost always have to shift around clauses in order for things to make sense. In my view, translating is first and foremost a test of reading comprehension. Translators need to first decipher authorial intent, and then find a way to articulate that intent in a completely different language in a way that makes sense to both them and readers. They need to determine what information the author is trying to push out as well as the general emotions and atmosphere that the author is trying to convey. But as long as it conveys the right intent with the right nuances, there's no real right answer in how to translate a line. (As an aside, the "authorial intent" portion is what I believe that the controversial industry translators at companies like CrunchyRoll or certain video game companies struggle with)
Regarding a "proper frame of reference", it's true that the literal country of Rome likely does not exist in this universe. But it doesn't need to, because the meaning of the idiom is much more important than the individual words within them. When someone says "It's raining cats and dogs", they don't mean that cats and dogs are literally falling from the sky. Likewise, we aren't referring to the actual country of Rome, but rather the underlying meaning of the idiom that almost exactly equates to how I would translate the original Japanese line literally. Again, this was a stylistic choice that Hax made and I agreed with purely based on our translation styles and philosophies. In this case, it would've been pretty easy to just slap on the literal translation, but I find that a little more boring.
I don't mean to say that your comment is wrong, per se. Hax and I have these kinds of conversations all the time, bringing up exactly what you pointed out. In particular, we have a running joke that Jesus Christ exists in Elite Knight's universe because we've used the term "Jesus Christ" as an expletive in a few chapters. But that is also a stylistic choice, and one that I've never heard anyone say anything about because the meaning of the expletive extends far beyond the literal words.
In conclusion, you make a valid point. But it's a point that Hax and I have already considered and decided to proceed anyway, because we didn't find it significant enough to dissuade our course of action considering our translation styles and the justifications made above.
Thanks for reading this rant this far if you made it here. It's just fun to write these rant-style essays detailing my thoughts on a matter that I find interesting. And the topic of translating is something that I personally enjoy thinking and talking about very much. I probably wouldn't be doing manga stuff if I didn't.
Edit: This would've made really good endpage material. Darn. Doesn't feel right to slap it on an endpage now that I've already posted it.