Translations in sourceLanguage does not work in Yii2 application - translation

Question

I'm using a translations based on keywords in my Yii2 application (I know, that this isn't best option, but I don't have other). I've prepared #app/messages/pl/app.php and #app/messages/en/app.php files with translation strings using keywords, instead of full-featured sentences or words:
<?php
return [
'name_english'=>'Name in English',
'keywords_english'=>'Keywords in English'
];
?>
I have set my application to use Polish language as default:
'language' => 'pl',
'sourceLanguage' => 'en',
And I'm invoking translation:
Yii::t('app', 'keywords_english');
Everything works fine, when language is actually set to base, Polish (pl):
But, when I change it to English (en; either by setting Yii::$app->language during runtime or by changing application configuration), translation is not performed and I'm getting keywords_english:
I have put die() in the beginning of #app/messages/pl/app.php and #app/messages/en/app.php files and I can clearly see, that when language is set to English, second file is not being included by Yii2 (application run follows), while, when language is Polish, first file is included and application flow breaks on that die().
What am I missing? Why Yii2 is not using translations from #app/messages/en/app.php file, if language is set to English (en)?
EDIT: By default, I was not altering default i18n component configuration in my application's configuration as I found no need for that. Translation files are stored in default position (#app/messages/<language>/) and are using default class (yii\i18n\PhpMessageSource). And this is working for all languages except sourceLanguage. At some point, I tried to alter configuration:
'i18n' => [
'translations' => [
'*' => [
'class' => 'yii\i18n\PhpMessageSource',
'sourceLanguage' => 'en',
'basePath' => '#app/messages'
],
],
],
But, it brought no change (why should it -- it still uses default settings).

Answer 1

According to samdark at Yii Forum, this is by design. Translations are not performed, if language = sourceLangage.
To workaround this, and force translations in this case, one must set forceTranslation to true.
Therefore, one must add / modify i18n component in components section of application's config in the way similar to this:
'i18n' => [
'translations' => [
'app' => [
'class' => 'yii\i18n\PhpMessageSource',
'forceTranslation' => true
],
],
],

Answer 2

Solution:
'translations' => [
'app*' => [
'class' => 'yii\i18n\PhpMessageSource',
'basePath' => '#app/messages',
'sourceLanguage' => 'en_US',
'fileMap' => [
'app' => 'app.php',
],
],
Answers for your comment:
1) 'sourceLanguage' => 'en_US' - You must use full locale. Because English locale may be en_US, en_UK and e.t.c. The format for the language/locale is ll-CC where ll is a two- or three-letter lowercase code for a language according to ISO-639 and CC is the country code according to ISO-3166. from [doc][1]
2) In key use category. And category set in Yii::t('category'...)