<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _make_unique_slug(slug: str, language: str, is_unique: Callable[[str], bool]) -> str: """Guarentees that the specified slug is unique by appending a number until it is unique. Arguments: slug: The slug to make unique. is_unique: Function that can be called to verify whether the generate slug is unique. Returns: A guarenteed unique slug. """


        index = 1
        unique_slug = slug

        while not is_unique(unique_slug, language):
            unique_slug = '%s-%d' % (slug, index)
            index += 1

        return unique_slug