How best to do language detection user?

Pliz tell me what is the most optimal way possible to determine what language to display the site.

Want to check in index.php and in the body of the page to display the JS variable

var language = 'EN';

Then in the script to initialize

var lang; var languages = { 'EN':{ 'qq':'Hello', 'fav':'Favorites', 'all':'All' }, 'en':{ 'qq':'Hello', 'fav':'favorites', 'all':'All' }, init : function(){ lang = this[language]; } }; languages.init();