Translations in your LocaleData projects can be created and edited in two ways - directly through the web interface, or by importing your existing YAML translation files.
You can import your translation files via the web interface. You just need to create the project language with the correct code (if it doesn't exist already), and then upload one YAML file at a time. The file is then processed in the background. During this process, the missing translation keys are created and the existing translations are updated. If any errors occur during the import process, they will be shown on the import details page.