system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] Geralt said he adopted the name 'Geralt of Rivia' because it sounded simple and in the books he even mastered a Rivian accent. Why then, do Eskel, Lambert, Vesemir, and Triss Merigold also speak with Rivian accents?

Asking again since I got downvoted yesterday.

In the books, people instantly recognise that Geralt as a foreigner from his Rivian accent. It is described as distinct from Temerian and Redanian accents. Triss Merigold hails from Maribor, a city in Temeria, so her accent should be fairly distinct from Geralt's despite it's portrayal in the games. The games treat the books as canon, so this should also apply to Geralt's character in the games.

Geralt also mentioned that he once wanted to take the name of Geralt Roger Eric du Haute-Bellegarde, which Vesemir dismissed as being too long. He then took up a Rivian identity. If witcher initiates had a choice in their identity, why do Eskel and Lambert also have accents similar to Geralt's? Do they like to pose as Rivians too?

In Blood And Wine, Prince Anseis of Lyria and Rivia also has an accent completely different from Geralt's. It even sounds more Nilfgaardian. Which of these two speak with genuine Rivian accent?
gpt
It's important to remember that Geralt is nearly a century old, and has spent most of that with Lambert.  It makes sense that he'd sound very similar.  Likewise with Eskel and Lambert, they've both spent decades with Geralt and Lambert.

That's also enough time for culture to shift in the ebb and flow of borders and battles and migrations.

There's many reasons why noteable figures might have different or similar accents, not least of which is fostering and tutors, and just plain old affectation.