def validate_nb(nb):
    """
    Validate that given notebook JSON is importable

    - Check for nbformat == 4
    - Check that language is python

    Do not re-implement nbformat here :D
    """
    if nb['nbformat'] != 4:
        return False

    language_name = (nb.get('metadata', {})
        .get('kernelspec', {})
        .get('language', '').lower())
    return language_name == 'python'