I think, I found the solution for it.
1) The code was already there, but since this sounded like a "new feature" request, I was just doing some research outside to see how it could be done, and didn't look a the code itself.
2) Today Rewarder pointed us to the fact that xLanguage supposed to be doing it already, so I look inside, and it seems that there were just bugs, which prevented xLanguage form auto-detecting users language on his first visit.
3) I think, I was able to fix the bugs, and we'll be testing on Rewarder's Website to see if it works.
But it will only detect the language with the highest score. Normally the browser provides in the:
$_SERVER['HTTP_ACCEPT_LANGUAGE'];
values like these, i.e. language code and the importance value:
de-DE,de;q=0.92,pl-PL;q=0.83,pl;q=0.75,ja-JP;q=0.67,ja;q=0.58,fr-FR;q=0.5,fr;q=0.42,it-IT;q=0.33,it;q=0.25,en-US;q=0.17,en;q=0.08
In Firefox there are extensions that can change that, but in Chrome I couldn't find an extension for it. I can change the language in settings and it will switch to it, but English will still stay with the highest value, since it was the original default.
If somebody knows how to extract the current language default in Chrome, please let me know.
Otherwise, xLanguage will detect the language with the highest score.
After the tests, I'll let you know how it worked....