Return string from promise result

I want to have an wrapper function arround this $translate service: https://angular-translate.github.io/docs/#/guide/03_using-translate-service so we can use those function easily in our code by calling $language.translate(‘keyword’) instead of the promise.

So, I create a new service in my app which should do that work. It’s a very simple function but it’s returning undefined or [Object Object].

 angularApp.factory("$language", ['$translate', function($translate){

    function trans(keyword){
        console.log("translate in $language", keyword);

        return $translate(keyword).then(function(msg) {
            console.log("translation successfull", msg);
            return msg;
        }, function(translationId){
            console.log("translation is not known", translationId);
            return translationId
        })
    }

    return {
        translate : trans
    }
}]);

In both cases, when translation is known or not, the console.log() shows me the right string, but on my page it’s showing [object object] or undefined (when I remove the first return right before $translate(keyword).
When I use the filter like {{:: "KEYWORD" | translate}} it works fine.

How can I let my function return the string with the translation or the translationId (which is actually the same as the keyword when there is no translation found)?

Source: AngularJS