If we want to wait for some server data for directive loading this is the pattern:

angular.module('busuuApp.translations')
    .directive('trs', ['trs','$http', '$q', 'wordbee', function(trs,$http, $q,wordbee,wordbeeStrings) {

  'use strict';

  var translateArgs = function(str) {
    try {
      if (str[0] != '"' && str[0] != "'") {
        return trs.call(null, str);
      } else {
        // Strips the " or ' on the start and end
        // Used to be eval("trs(" + str + ")");
        return trs(str.slice(1, str.length-1));
      }
    } catch (err) {
      $log.error('Reference error, trs directive shouldn\'t have dynamic vars: ' + args);
      throw err;
    }
  };

  //store the data so you don't load it twice.
  var directiveData,
  //declare a variable for you promise.
      dataPromise;

  //set up a promise that will be used to load the data
  function loadData(){

    //if we already have a promise, just return that
    //so it doesn't run twice.
    if(dataPromise) {
      return dataPromise;
    }

    var deferred = $q.defer();
    dataPromise = deferred.promise;

    if(!_.isEmpty(wordbeeStrings)) {
      //if we already have data, return that.
      deferred.resolve(directiveData);
    } else {
      console.log("TRS directive loadlanguage");
      wordbee.loadLanguage(interfaceLanguage)
                  .then(function(data) {
                    directiveData = data;
                    wordbeeStrings = directiveData;
                    _.each(wordbeeStrings, function (val, key) {

                      val = val.replace(/&lt;/g, '<');

                      val = val.replace(/&gt;/g, '>');

                      wordbeeStrings[key] = val;

                    });


                    console.log("trs directive Language loaded!!!");
                    deferred.resolve(directiveData);
                  });
    }
    return dataPromise;
  }

  return {
    restrict: 'EA',
    scope: false,
    link: function(scope, elm, attrs) {
      //load the data, or check if it's loaded and apply it.
      loadData().then(function(data) {
        //success! set your scope values and
        // do whatever dom/plugin stuff you need to do here.
        // an $apply() may be necessary in some cases.
        //console.log("TRS directive --> ",attrs);
        if (attrs.hasOwnProperty('trs')) {
          elm.html(translateArgs(attrs.trs));
        } else {
          // otherwise we use <trs> </trs> syntax
          elm.replaceWith(translateArgs(elm.text()));
        }
      }, function() {
        //failure! update something to show failure.
        // again, $apply() may be necessary.
        scope.data = 'ERROR: failed to load data.';
      });
    }
  };
}]);