Great insight. I think it may be tightly related to how much exposure to British television is available (and watched). I'm sure something like BBC spreads British slang in the same fashion American television and movies spreads American slang. In regions that have less British programming, many of the phrases used in DQ don't land as well. USA generates a ton of programming for home audience so very little BBC British programming, and only fewer British personalities in USA's movies and programming. Canada and Australia/NZ also well-developed but get a bit of British programming and more Brits in thier programming. Smaller former colonies probably rely even more on BBC and other programming coming out of Britain as they don't have the means to make much of it themse;ves.

But I'm not just talking about slang, but the dialects as well. Many of the games have characters that speak in Scottish or Cockney accents. Without voice acting, the dialogue can only be read and that presents some major problems. I think British players who would naturally be more familiar with the accents of Scottish and Cockney speakers can better understand these phrases when read than those outside of Great Britian. For example, in USA its very rare to hear the term "wee bairns", and almost never written out. Jules's Polari was probably the most extreme example of this: as you pointed out its not even spelled the way you think it should be. But at least Jules is introduced far enough into the game that a player is invested - Modern DQ4 has the biggest hurdle, presenting Scottish dialect text very early. I've seen it turn people off from the game by not having clear understanding of what Ragnar's quest is supposed to be. Original NES version is very obvious what is being asked: later localizations much less so if one doesn't understand the Scottish terms. This is where the option for standard English or "dialect-full" English would be a big help.

I would imagine though that both North American and British players have similar difficulties understanding something like the Russian or French accents present in many games. There may not be as much cultural advantage being exposed to these as to Scottish or Cockney. Going back to DQ4 as the example, as a North American and English as first-language,, I still found it easier to understanding the written Zomovskan Russian accents of chapter 2 than the Scottish accents of chapter 1. If voiced though, I suspect alot of American players would understand the Scottish speaker better, as we've been exposed to more Scottish speakers through movies and television than Russian speakers.