Added custom language code to config schema (#3542)
Added custom language code to config schema (#3542)

As suggested in https://squidfunk.github.io/mkdocs-material/setup/changing-the-language/#custom-translations
squidfunk/mkdocs-material,squidfunk/mkdocs-material,squidfunk/mkdocs-material,squidfunk/mkdocs-material