That’s actually not a bad idea for a template function.
lang.LanguageName SRC [TARGET]
If TARGET is not provided, set to SRC.
{{ lang.LanguageName "pt-br" "en" }} --> Brazilian Portuguese
{{ lang.LanguageName "pt-br" "pt-pr" }} --> português
{{ lang.LanguageName "pt-br" }} --> português
And you could pass in {{ or site.Language.LanguageCode site.Language.Lang }}
.