Adding new project locales
If you have permission to add locales to a project, you will see a "New locale" button with an :add icon: in the locales panel of your project management screen.
In the "New locale" modal window you can choose from hundreds of common locales. Just start typing and the field will auto-complete.
If you don't see the exact locale you need, switch to "Custom" mode to choose from thousands more languages or create the specific variation you need. See Creating custom locales.
If you know the exact short code you need, switch to "Advanced" mode and enter any valid language tag. See Developer locale support.
Modifying project locales
If you have permission to edit locales in a project, you will see a :cog icon: next to each item in the locales panel of the project management screen. Click this icon to modify the locale properties as if you were creating a custom locale.
See custom locales for the fields you can change here.
Switching source locale
When you start a new project your source locale is added automatically. This defines the primary language you are translating from.
You can modify the source locale like any other, but you can also make one of your other project locales the source at any time. See project settings.