Dynamic concatenation of a variable in jQuery

You can use the JavaScript brace notation [] to access a variable with a variable name:

// declare some global variables
var foo = 'foo value';
var bar = 'bar value';
var baz = 'baz value';

// <parenthesis>
// For information, the line below (when executed in global scope ONLY):
var x = 'x'
// is equivalent to this:
window.x = 'x'
// </parenthesis>


// choose what variable you want to use
var varName = 'bar';


// use 'varName' to retrieve the value
var theValue = window[varName];
console.log(theValue);

// here's the step by step of what's happening
theValue = window[varName];
// equals to
theValue = window['bar'];
// which get executed as
theValue = window.bar;
// which is the same as (see parenthesis above)
theValue = bar ;
// then finally
theValue = 'bar value';

So, you're problem could be solved as follow:

// declare some global variables
var enMain  =   "Main Text English";
var enSub   =   "Sub Text English";

// choose what variable you want to use
var prefLang = 'en';
var varName = prefLang+'Main';

// use 'varName' to retrieve the value
var mainTitle = window[varName];
console.log(mainTitle); // "Main Text English"

However, I think you're doing some internalization process, and without the help of external libraries, I would do it that way:

// step 1) declare all your texts

var availableTexts = {
};

availableTexts.en = {
    Main: "Main Text English",
    Sub: "Sub Text English",
};
availableTexts.es = {
    Main: "Texto principal Español",
    Sub: "Texto secundario Español",
};
availableTexts.fr = {
    Main: "Texte principal en français",
    Sub: "Texte secondaire en français",
};
// and so on...


// step 2) get the value from URL
// I hard code it here just for the demo
var prefLang = 'en';
// define a fallback language if the language
// you're looking for does not exist
var defaultLang = 'en';


// step 3) retrieve all the values, localized according to prefLang value
// if not found, fall back to the values of defaultLang
var localizedText = availableTexts[prefLang] || availableTexts[defaultLang];


// step 4) in all your code, you just have to use the variable localizedText
// now you don't care what the user language is,
// and you keep your code so dead simple by the way !
$.getScript("/js/bannerText.js", function(){
    console.log(localizedText.Main); // "Main Text English"
});