This is about to get a little rambly and I apologize in advance.
I really loved hearing the return of Icelandic accents for Viis/Viera NPCs (and all but one of those voices were from actual Icelanders! Good work SE!), and the Ronso/Hrothgar having their own unique accents (I jokingly refer to them as "vampire accents" but I'm having trouble pinpointing the exact brand of Slavic they were going for tbh) was a nice touch as well.
On the other hand, it does sort of make the NPCs on the Azim Steppe feel like a missed opportunity, imo. And it's not that all the NPCs in Stormblood lacked authentic accents; Yotsuyu and Hien both had a noticeable, if faint, Japanese accent, most noticeably on certain specific words ("Doman" for example). I know it's not realistic to expect SE's English voice studio of choice to be able to pull in actors with every accent imaginable, but I do feel like it might have been a nice touch for immersion's sake. The usage of correctly-pronounced Mongolian terms was pretty great, though.
I also think it's a bit of a shame that despite the obvious French influence on Elezen names, there isn't really a whole lot of French influence reflected in their mode of speech. A lot of the pronunciations also aren't really accurate or consistent, Edmont and Haurchefant's names being good examples.
Anyway, overall I think the localization team has done a great job of working regional accents, dialects, and foreign-language vocabulary into the game. If anything I'd be curious to see more discussion of accents and native languages in future lore books. Maybe even something related to the way the written language of Norvrandt is so different from that in Eorzea.