setCookie.js

var 

setCookie = function (c_name, value, exdays) {
	var exdate = new Date();
	exdate.setDate(exdate.getDate() + exdays);
	var c_value = escape(value) + ((exdays == null) ? "" : "; expires=" + exdate.toUTCString() + "; path=/");
	document.cookie = c_name + "=" + c_value;
};

setCookie("BsiLanguage", "pl_PL", 90, "/");

document.cookie = 'BsiLanguage="pl_PL";" is the fastest way to add a cookie.

Ah sorry, second double quote must be single. Fixed code is below.

document.cookie = 'BsiLanguage="pl_PL";';