Translating texts, that appear only in one language

I have a dual-language (English and Polish) application written in Yii 1.x. Base langauge is English (sourceLangage) and current language (language) is controlled by user.

How should I translate, using Yii::t, strings, that are not used in English, but should be visible in Polish?

I can’t call Yii:t(‘app’, ‘’);, right?

Just use Polish. Not found strings are not translated.

Yii::t(‘app’, ‘Gżegżółka’);

If you want the English version to disappear add in messages/en/app.php

'Gżegżółka' => ''

or detect language and add some conditional viewing mechanism.