Accents have everything to do with your environment and surroundings during early development and have nothing to do with genetics.

For example, I'm from London, England. Virtually every race and ethnic group lives here and we all speak with the same accent (excluding migrants who grew up elsewhere, of course). Meanwhile people with long English ancestry who grew up in Manchester, for example, sound completely different.

Tural isn't a real world location, they don't need south American accents - they don't have them in other adaptations, so why do they need it in English only?? Japan are making their own fantasy rules like "Turali speak with Osaka accents!" (for example, I dunno), so why couldn't Turali speak with Icelandic accents in English, considering the only native Turali we knew pre-DT speaks like that? Surely the English studio located in the UK could have more easily flown Icelandic actors in and given them proper direction.

If they cared for browny points so much, they could have properly invested in scouting and hiring actual native South American speakers and flown them to the UK for the job.

I dunno, their approach in DT is so half-baked and disjointed to the point everyone speaks with different accents, and ones that sound super fake at that.
I don't blame the VAs themselves, as a series of weird decisions led to this bad outcome.