There is no need for you to manually create and resolve a defer on your own. There is actually a simpler method available. (It's worth noting that in the example provided, the promise is resolved before the underlying ajax call is completed)

It's interesting to note that Angular.js $http service also adheres to the $q interface! You can achieve the same functionality with the following code snippet:

app.factory('customLoader', function($http, $q, SERVER_URL) {
  return function(options) {
    var promise = $http.post(SERVER_URL + 'getLocaleData')
      .then(function(localeDataBag) {
         var localeData = localeDataBag.data; // The '.data' property of 'then' is equivalent to what '.success' would return.
         modifiedLocaleData = localeData++;  // Make modifications to the local data here
         return modifiedLocaleData; // This is the updated result
        })
      .catch(function(err) {
        console.log(err);
        return $q.reject(err); // This is the new result in case of any failures. It's important to handle failure states when chaining promises.
      });
    return promise;
  };
});

JavaScript operates asynchronously, meaning that the final console.log will execute before the server has returned any data. Therefore, you must include it within both of your promises:

$http.post(SERVER_URL + 'getLocaleData').success(function(localeData) {
    console.log(localeData);
    }).error(function(err) {
         console.log(err);
        alert('warning', err.message); //It is assumed here that your server returns a JSON object with a "message" property
    });

This way, the server response will be logged using console.log in both success and failure scenarios.

Reference

Edit: Alternatively, if you prefer to approach it differently:

var response; //Begin by declaring a variable in the parent scope.

$http.post(SERVER_URL + 'getLocaleData').success(function(localeData) {
    response = localeData; //Assign the response data to this variable.
    console.log(localeData);
    }).error(function(err) {
        alert('warning', err.message); //It is assumed here that your server returns a JSON object with a "message" property
    });
    console.log(response);

However, please note that the example may not behave as expected. The final console.log statement will likely output undefined.

To access your server response in the controller from outside the factory, you can follow this method:

app.factory('customLoader', function ($http, $q) {
        return {
            response: {},
            getLocalData: function (param) {
                var deferred = $q.defer();
                $http.post(SERVER_URL + 'getLocaleData', param).
                    success(function (data, status, headers, config) {
                        deferred.resolve(data);
                    }).
                    error(function (data, status, headers, config) {

                    });

                return deferred.promise;
            }
        }
    });


 app.controller('yourCtrl', function($scope,customLoader) {

       $scope.GetLocaleData= function() {    
                customLoader.getLocalData({
                    Role: role,
                    Email_Id: $scope.Email,
                    Pwd: $scope.Password
                }).then(function(localeData) {
                    $scope.Data = localeData;                        
                });        
        }
 }); 

localeData is confined within the scope of the .success function. To access it outside, you need to assign it to another variable.

var data = ""
$http.post(SERVER_URL + 'getLocaleData').success(function(localeData) {
    data = localeData;
    }).error(function(err) {
        alert('warning', err.message);
    });
//need to obtain that value here.
console.log(data);

However, don't expect to retrieve the desired value in data until you receive a response back. The $http.post function operates asynchronously.

To address such issues, consider looking into promises, as they are commonly used in the AngularJS world.

UPDATE:

If you prefer, you can utilize the $q service like this:

app.factory('customLoader', function($http, $q, SERVER_URL) {

    return function(options) {
        var deferred = $q.defer();

        // perform actions with $http, $q and key to load localization files
        $http.post(SERVER_URL + 'getLocaleData').success(function(localeData) {
         //perform actions on localeData here
        }).error(function(err) {
            alert('warning', err.message);
        });

        deferred.resolve(localeData);
        return deferred.promise;

    };
});

Alternatively, you can simply return the $http.post itself, as it returns a promise:

app.factory('customLoader', function($http, $q, SERVER_URL) {
    return function(options) {        
        return $http.post(SERVER_URL + 'getLocaleData');
    };
});

In either case, a promise object is returned rather than just the actual server response. You can access the localeData like this:

customLoader().success(function(res){
   console.log(res);
 }).error(function(){
    //display an error message
 })