Both stars struggled with a long-lived movie heritage in which accents flourished less as a precise locator of birthplace or background, more as a primal colour on the dramatic palette. Garbo's husky Scandinavian mezzo enhanced her otherworldly aura as a woman who would die for love, or cause her men to do so. Charles Boyer's bedroom-voiced French was a charmer's charter, a licence to seduce. The cast got their dimwit British rockworld brogues so right that you couldn't tell spoof from truth. But with rules there are always exceptions, even to the one that orders us to rejoice over accent mastery.
Examples are myriad.
An actor can become a star by playing against grain or inclination, against pedigree or plausibility. And a bad, stagy or surreal accent can be the passport to a great iconic event, proving that we sometimes prefer inspired perversity to dull accuracy. Anthony Hopkins as Hannibal Lecter. Hopkins's accent belongs absolutely nowhere, except on some imaginary atoll in mid-Atlantic. As for the future, the next great test case for glottal globalism arrives soon.