Loco Translate allows you to create language files in any language. If your language does not show in the list of “Available languages”, it means there is no community GlotPress project in your language.
Selecting “Custom language” in Loco Translate lets you enter any valid language code, whether the language is known to GlotPress or not. See Adding a language > Choose a language.
For your new language to show in the list of “Installed languages” you must have a core translation file at wp-content/languages/{code}.mo. You can use Loco Translate to create that file under the WordPress section of the submenu. I recommend if you do this, you install a known language file from GlotPress and copy it to your language. It’s not advisable to generate your own core templates.
