The devs wanted to make the Sith Empire seem like Palpatine's Empire to keep everything in terms of a Star Wars that even casual fans could understand quickly. So they went with tropes that they thought people associated with Palpatine's Empire for the Sith Empire: the design of their spaceships, the cut of their military uniforms, even the emblem.
I really, really doubt they'll end up doing anything so drastic, but between the emblem and accent of the Sith Empire both later being such prominent parts of the Republic, I like to wonder if maybe this war doesn't end the way everyone assumes it will
It's one of those things that doesn't really bother me that much from a lore perspective though. It's not like Galactic Basic and English are actually the same language, so the accent itself would be as much a translation convention as the underlying language. For purposes of "translating" during this period, "British accented English" = "DK accented Basic", during the movie time-frame the translation becomes "British accented English" = "Coruscanti accented Basic".