Language selector pattern

[ffoodd]

Language selector is a common pattern, used in Orange navbar, some example pages and in incoming essentials global headers…

This pattern should be harmonized and, obviously, accessible:

• [ ] use full word for language with the right lang attribute, if it's impossible (design or UI constraint), use visually the two digits language code with a aria-label=language code (two digits in capital letters)+ " " + the full language word (capitalize the first letter), example "FR Français"
• [ ] use targetted language word as content, with appropriate lang and hreflang attributes
• [ ] show current language, as another dropdown-item (with .active and aria-current="page")

[ffoodd]

Should probably be documented on a11y-guidelines first (ping @GaetanBt) then shown in some Boosted examples.