How can I easily translate standard buttons (Yes, No) from QMessageBox? I can't use self.tr on those arguments, so I would like to achieve it in some other simple way. Do I have to use whole translation system?
If you want to have Yes/No translated into the current language, usually setting the system language should be enough. Otherwise try with Natim's suggestion. 
If you want custom texts: The static methods like question() etc. don't allow custom button texts. That's unfortunate, as "Yes"/"No" dialogs are considered bad style (they are not descriptive, one has to read the whole message, and its easy to misunderstand what means Yes and what means No and accidentally negating them). For custom texts, you have to use QMessageBox directly. I'd suggest to write your own static methods accepting optional button texts.
PyQt: translating standard buttons