You need to reference the module name, so say you have a Page module(/modules/page) and your translation for that module is located in /modules/page/messages/LNG/page.php, you will call Yii translation like:
Yii::t('PageModule.page', 'string to be translated');
Basically, the signature is:
Yii::t('ModuleNameModule.fileWhereTheTranslationCanBeFound', 'string to be translated');