var lang = (navigator.language || navigator.systemLanguage || navigator.userLanguage || 'en').substr(0, 2).toLowerCase();
And usage example:
var lang = (navigator.language || navigator.systemLanguage || navigator.userLanguage || 'en').substr(0, 2).toLowerCase();
if (lang == 'ru') {
document.write('Specific Russian text');
} else {
document.write('Common English text');
}