Getting 'Preferred language' value

Getting 'Preferred language' value

Good day,

How to and in which memory variable read the 'Preferred language', and make another HTML page call based on this value ? A HTML instruction, a JavaScript or a jQuery instruction ? Which one ?

'Preferred language' is in fact yet memorized in ...
- Mozilla FF browser : Via Menu's : Tools > Options > Content > Languages
- IE browser : Via Menu's : Still have to find the path
- Google pages : in 'Search parameters' (up right)
- in the OS parameters (registry keys) of Microsoft.

I assume that as well for Google as for any brand and type of browser, this 'preferred language' is memorized in a 'Registry key'.

Can anybody tell me if this is a right way to jump to a set op pages of a given language ?
Where to find basics theory to learn, an example of code or a forum discussion on this topic ?

To me, choosing between different HTML pages based on a value anywhere in memory should be a very common issue, isn't it ?

Thanks for trying to help me.