Yii doesn’t support locale inheritance (i.e., ru_ua inherits ru). For this reason, you need to set your language ID to be ‘ru’ in order to use the existing Yii core messages. If ru_ua is a requirement, you may refer to the following tutorial to resolve your problem:
I like your approach naming a language with 2 chars (e.g. ‘ru’) rather than my approach with 5 chars (‘ru_ua’), and ‘ru_ua’ isn’t a requirement of this project.