I can confirm that on Google Chrome, the first preferred language will be the UI language shown.

That man is very informative about what he is doing! Anyway, I also found this link (made by the same guy in the YouTube video) https://www.hackingwithswift.com/100/45 That isn't exactly a full enhanced tutorial, but gives a beginner like me a better idea of how to make something. Thank you for pushing me in the right direction!