> So if it took the printing press to encourage standardisation of written English (I can only speak for Britain), it took
another invention (the radio) to begin the standardisation of spoken English.

This is very interesting. There was a programme on BBC last year about 100 years of cinema (or something) and the producers interviewed a number of English people who had been to see the first talkie 'The Jazz Singer'. Previous to this the British audience had seen only silent American films.

One English girl recalled that she had never heard the American accent before and neither she nor her friends could understand a word that Al Jolson siad for the rest of the film. Watching the same film today it is laughable to think that this could ever be the case because we have become acclimatised to others' accents through broadcasting and media but, back then, American English was a foreign language and was just as difficult to understand as spoken English in one area was to someone living in a far corner of England. So the radio standardised spoken English whilst the movies (and later television) standardised global speech. Local dialect can be heard interspersed with snippets of foreign slang on every street, in every major city in the world but I believe that this foreign slang is itself being turned into a unique form of local dialect.

An example. In the US it is common (or uncommon) to say 'say what?' In Dublin you would hear a variation - 'you what?' which means exactly the same thing. But 'you what?' has only become Dublin parlance in the past 20 years - clearly a fallout from TV.