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Why can't English be as stable as other languages?
Probably because it's the most widespread of all the world's major languages.
The French may try to regulate the use of their language by evoking the authority of the Academie, but with little real success.
'Le weekend', 'le parking' and 'Le hamburger' will prevail.
I overheard two French speakers talking on a bus recently; my French is not what it was, but I definitely made out 'du feedback' and 'des emails', pronounced 'a l'anglais'.
(apologies for the missing accents. I don't know how to get them)
I think 'American' English has been particularly prone to change over the years because of the diversity of languages spoken by migrants to that country. German speakers, Italian speakers, Spanish speakers and speakers of a plethora of Eastern European and Oriental languages have all adopted English as the lingua franca, but with some modifications.
Example: where did 'different than' come from? It's not used in UK ('different from' or 'different to' are standard, but not 'different than').
Is it perhaps a literal translation of German 'anders als', which was used by German-speaking migrants and then spread through North America?
'I saw it already.' Most certainly not correct in UK English where 'already' is only used with a present or past perfect tense ('I have already seen it'), but not with a past simple.
Another translation? From (I'm guessing here) an Eastern European language, or from Yiddish?
It is, of course, 'American' English which is now spreading round the world by means of cinema,TV and the social media. Consequently 'British' English, 'Australian' English, 'South African' English and all the rest of what started in the seventeenth century simply as 'English' are being transformed into something quite different.
It's normal; Latin evolved into French, Italian, Spanish and so on, after the fall of the Roman Empire. The British Empire has gone - so has the language of Victorian England.