The name "Duval" is, obviously, of French origin, but the out-of-lore origin of the name is simply chance. The original FD lore was largely written out of whatever the Stellar Forge's ancestor-algorithms happened to generate in the FE2 universe. The algorithms were pre-programmed with a bunch of names, "Duval" was one of them, and the name was scattered throughout human space. The lore-writers chose it as the name of the Imperial family, perhaps because several prominent Imperial worlds had "Duval" in their name.
In-lore, there is no specific mention of the Imperial family's Earth-ancestry. The lore is that the founders of the Achenar colony fled as far away from the suffocating rule of the Federation as possible; Achenar was one of the most remote Earthlike worlds known at the time. The ethnicity of the Duvals or other founders was never mentioned in lore.
As for the Imperial language/dialect, the continuum of Elite lore has not been consistent. In the old FE2 lore, possibly retconned by now, it states that the upper-crust Achenar accent was created specifically by imitating and mocking the standard Federation accent of the day. As such, it doesn't really sound like any 21st-century Earth accent. "To the foreigner their speech sounds rather like a whine", the Gazetteer lorebook states. I've heard it said that the Australian accent sounds rather whiny to foreigners, so perhaps the Imperial accent could be best recreated by getting an Australian to do a bad job of imitating the American accent - or perhaps vice-versa.
This is the Achenarian accent, which would dominate Imperial culture for a millennium and which the ruling elites of the colony worlds would inevitably attempt to copy. However, immediately after defeating the Federation the Empire expanded rapidly via military conquest, and the assimilated colonial cultures would have brought their own interpretations of Imperial dialect into the melting-pot.
In FFE, the video clips that played while on Imperial stations featured a voice actor who always tried to sound both angry Space- and hyper-posh British; perhaps this was merely to try to distinguish his Imperial characters from the Federation and Indie/Alliance characters he also played and which were done in a more normal British voice.
Finally, you have the voices from Imperials as you hear them in ED. The Imperial station-announcers are Anglo-Japanese, while their traffic controllers sounds upper-class British (to my Australian ears, anyway). And lastly, you have the recordings of members of the Imperial Family themselves in the Codex, which mostly just sound normal-British-accented to me.
