Great article from Wictor Wilén regarding that topic: Announcing new Visual Studio 2012 tool for JavaScript Localization in SharePoint 2013
Short sample code:
function requestUserPolicyDialog() {
SP.SOD.executeFunc("sp.core.js", null, onSpCoreReady);
}
function onSpCoreReady() {
SP.SOD.executeFunc("sp.js", null, onSpReady);
}
function onSpReady() {
SP.SOD.registerSod("MyStrings", "/_layouts/15/ScriptResx.ashx?name=MyStrings&culture=" + STSHtmlEncode(Strings.STS.L_CurrentUICulture_Name));
SP.SOD.executeFunc("MyStrings", "Res", onResourcesReady);
}
function onResourcesReady() {
alert(Res.someString);
}