code: def select_locale_by_request(self, request, locales=()):
          
          default_locale = locales and locales[0] or self.cfg.default_locale
  
          if len(locales) == 1 or 'ACCEPT-LANGUAGE' not in request.headers:
              return default_locale
  
          ulocales = [
              (q, locale_delim_re.split(v)[0])
              for v, q in parse_accept_header(request.headers['ACCEPT-LANGUAGE'])
          ]
          ulocales.sort()
          ulocales.reverse()
  
          for locale in locales:
              for _, ulocale in ulocales:
                  ulocale = locale_delim_re.split(ulocale)[0]
                  if ulocale.lower() == locale.lower():
                      return ulocale
  
          return ulocales[0][1]
docs: Choose an user's locales by request.