def _language_validator(l: T.List[str]) -> T.Optional[str]:
    """Validate language keyword argument.

    Particularly for functions like `add_compiler()`, and `add_*_args()`
    """
    diff = {a.lower() for a in l}.difference(compilers.all_languages)
    if diff:
        return f'unknown languages: {", ".join(diff)}'
    return None
Validate language keyword argument.

    Particularly for functions like `add_compiler()`, and `add_*_args()`
    
mesonbuild/interpreter/type_checking.py