if lang is None:
        return None
    lang = lang.lower()
    return lang


def complete_existing_lang(incomplete: str):
    lang_path: Path
    for lang_path in get_lang_paths():
        if lang_path.is_dir() and lang_path.name.startswith(incomplete):
            yield lang_path.name


@app.callback()
def callback() -> None:
    if is_mkdocs_insiders():
        os.environ["INSIDERS_FILE"] = "../en/mkdocs.insiders.yml"

                            langSet.add(normalizeLang);
                        }
                    }
                }
            }
            if (langSet.size() > 1 && langSet.contains(Constants.ALL_LANGUAGES)) {
                return new String[] { Constants.ALL_LANGUAGES };
            }
            langSet.remove(Constants.ALL_LANGUAGES);
            return langSet.toArray(new String[langSet.size()]);
        }