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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_default_locale_callable():
 """ Wrapper function so that the default mapping is only built when needed """

    exec_dir = os.path.dirname(os.path.realpath(__file__))
    xml_path = os.path.join(exec_dir, 'data', 'FacebookLocales.xml')

    fb_locales = _build_locale_table(xml_path)

    def default_locale(request):
        """
        Guess an appropiate FB locale based on the active Django locale.
        If the active locale is available, it is returned. Otherwise,
        it tries to return another locale with the same language. If there
        isn't one avaible, 'en_US' is returned.
        """
        chosen = 'en_US'
        language = get_language()
        if language:
            locale = to_locale(language)
            lang, _, reg = locale.partition('_')

            lang_map = fb_locales.get(lang)
            if lang_map is not None:
                if reg in lang_map['regs']:
                    chosen = lang + '_' + reg
                else:
                    chosen = lang + '_' + lang_map['default']
        return chosen

    return default_locale