Korean is my first language. These are just my personal observations:
In English, the word 'Eliminate' has multiple meanings depending on the context, including the meaning 'to get rid of/kill someone'. So when the second clause of the player consent form states that "A player who refuses to play will be eliminated", there is an immediate sense of uncertainty on the exact meaning. But the Korean word used here is 탈락/tallak, which is used exclusively to mean "fail in/drop out of/be eliminated from a contest/tournament". With that in mind, it is understandable (story-wise) that the players had less reason to suspect the deadly nature of the games when signing the form.
I think the first scenes with Sang-woo and Ali in episode 2 did a good and efficient job in fleshing out Ali's character as a migrant worker. When Sang-woo says they're at Yeouido, Ali asks where Yeouido is. Yeouido is one of the main financial districts in Seoul, so his lack of knowledge of such a common location tells the viewer that he barely stepped out of the area he lived/worked in (I think Ali said he lived in Ansan, which is on the outskirts of Seoul). Initially, he also repeatedly refers to Sang-woo as Boss, until Sang-woo gets uncomfortable and tells him not to call him that. Compared to English, the Korean language has a different and quite complex system to refer to one another. This may be a part of the reason why it's common for migrant workers like Ali to just stick to calling people Boss, 사장님/Sajangnim in Korean, to avoid any potential misunderstandings (and the boss is probably one of the few Koreans they have regular contact with). Later on in the games as they got closer, Sang-woo tells Ali to call him 형/Hyung, which has the standard definition of 'older brother', but it's also used between male friends who are close. And it is all the more tragic when the betrayal occurs, and Ali's last words were calling for Sang-woo 'Hyung'.
Saebyeok is from North Korea, but she has no noticeable North Korean accent during the series. Only in brief moments, such as when she was talking to her younger brother and the broker, are there some hints of an accent. There's only two moments that I can think of where she highlights her North Korean background. One is when she's talking to Deok-su before Game 2: "You know what we call people like you in my hometown? A revolutionary son of a bitch". Don't know if it's an actual North Korean term, but it sure sounds like one from a South Korean perspective. Another is in Episode 6 when she's talking with Ji-yeong about wanting to visit Jeju Island because it looked exotic and nothing like Korea. Here she uses the North Korean term to refer to Korea, which is 조선/Joseon (Korea is called 한국/Hanguk in South Korea).
Whether it's to avoid getting discriminated against, or to avoid the barrage of questions people would ask to a person from North Korea- North Koreans mostly adapt the South Korean speech and mannerisms so they don't stick out like a sore thumb. This is particularly true among young people. I don't know how long Sae-byeok has lived in the South, but no doubt she would have quickly adapted along with her street smarts. Thus her flawless standard Korean accent throughout the series doesn't detract at all from the immersion IMO.
That is all I have for now. Thanks for reading.