The UI of an application is designed for a particular language. This is called the default language, and it is in this language that the application is executed by default. You may have your application translated and executed in other languages, making it a multilingual application with a multilingual UI.
Use the OutSystems built-in mechanisms to localize your Traditional Web Applications and learn more about the end-to-end Traditional Web Application localization process in this Master Class on Multi-Language. For mobile apps, use the Multilingual Mobile Component from Forge.