How to add an unsupported language
If you want to translate to an unsupported language, you can translate the UI elements.
Create an i18n file To do so, you have to create the i18n diretory inside the root of the site, the directory where you can find the hugo.yaml file, and directories like data, content, etc.
Afterwards, you can create the file <language_code>.toml into the i18n directory. In this directory, you can find all language codes with the flag that will appear with that code.