Translating Core Error Messages
Agile Toolkit channels all the core exceptions through $obj->exception() function. This function passes it's argument to $api->_() method which is designed to provide translation for messages. By default all the text is returned as-is, however you can either use add-ons or define your own translation rules.
Here is example addon translating Agile Toolkit into russian: https://github.com/atk4/atk4-addons/blob/4.2/russian/lib/Controller/Translation.php