To add a new language to your project, simply click the Add new button in the top-right corner of the Languages tab.

This will open the Language Settings window where you can configure the new language's details.

In this window, you can configure the core and visual settings for a new language.
Language — The new language you want to add to your localization database.
Fallback language (optional) — If a translation is missing for this language, values from the fallback language will be used instead.
If none is selected, the default values will be used as fallback.
These are purely visual and do not affect functionality:
Click Add Language to finalize.