This article discusses the translation mechanism in Magento 2.x, and contrasts it with that of Magento 1.x. The intended audience is Magento developers who would like to better learn the Magento architecture.
The Magento 2.x source code described here has been available since Magento 2.0.0.0-dev46 release and can be viewed on the GitHub at https://github.com/magento/magento2/commit/0849373d7636cfa17b70b8188bfc36d13ae749c1 or later commits.
Magento 1.x can present the user interface in different languages without modifying the actual application source code—it translates the system messages, error messages, and labels for display in the UI. Some messages may be displayed in logs for a system administrator or a developer—those don't need to be translated. By convention, in the source code, the labels and system messages for UI are expressed in English (en_US).