No movie of Romulus and Remus has ever been made before. FYI, most of the "mythic" elements, like being suckled by a she wolf, are not included. Rather, he is placing the story in a very realistic context of shepherds trying to found a new village. What the director is doing, according to reports, is quite complex. It's not some nationalistic origin story.
Such movies have certainly been made in northern European contexts, of course. Look at the movie based on the Beowulf poem, or all the Thor incarnations.
What if an actor like Daniele Liotti had played one of the lead characters in Beowulf, or Joe Manganiello played Thor or Vercingetorix, for that matter? Personally, Manganiello would certainly appeal to me more than Helmsworth.
However, there would have been an outcry, and you know it, including from you. Would you think, well, it's just a myth, so who cares? I doubt it.
Now, as you say, a lot of this is marketing, and knowing your audience, but not all of it. I think you'd object if Manganiello played that character, even if it was an Italian movie marketed mainly to Italians and Southern Europeans.
Also, just generally, how much suspension of disbelief should a director demand of the audience, no matter the nature of that audience? Are Anglo and Northern European audiences so clueless or so tied to Nordicist myths about Romans that they'd have a problem with Romans who look like Italians? That's sad if true.
Also, in this day and age, movies are international. It's not the 60s. I think a little verisimilitude would be welcome. American movies engender criticism as well as high box office, you know.
There's one auteur who tries to do it right to the extent possible, and that's Mel Gibson. The actor who he cast to play Jesus in "The Passion of the Christ" was altered to look more "East Med", and the other actors were chosen with an eye to what contemporary Judeans and Romans would have looked like. It all contributed to making a better movie, a masterpiece, imo.
https://www.youtube.com/watch?v=Cm_-kNaPsp4
Likewise, in making a movie about Amerindians, he did a novel thing: he cast Amerindians, not Italians, as Hollywood movies used to do. It's a very good film imo, if not for everyone.
https://www.youtube.com/watch?v=pXuwjdQx924