If the target language is en, no translation will take place. If the target language is de, translation will occur. If you want a translation to occur every time, you can set the sourceLanguage to 00.
Check out this great wiki for more info Translations.
Also, I don’t think using ‘yii’ as your translation category in Yii::t(‘yii’, ‘test’) is a good idea, I think this is a reserved word for the translation method.