I laugh cause there was a research in Greece at experimental schools 3 times,
1 was about 50;s
1 about 70's
1 about 90's
consider that at 50's school had an artificial language
at 70 had thracian idiom (con/polis Greek Τρανταφυλιδης Γραμματικη 1955)
at 90 the modern Greek (free language that follows Thracian idiom's grammatical rules)
research was on 12-13 years old that first time learn Ancient Greek
the research contained Homeric Attic and Hellenistic2 (1100 BC 400 BC 100 AD)
at decate of 50's 70% understand hellenistic 45% Attic and only 25% homeric
at decate of 70's that was 85% Hellenistic 60% Attic and only 15% Homeric
at decate of 90's that was 60% Hellenistic 35% Attic and only 2% Homeric
the search was about words or forms that are not in common use every day
As you see in 3000 years first written and known Language become unknown to young people,
and they learn it in school as 'foreign' language,
the most exciting is that in 40 years less young people know hellenistic even though they go to church where testimony is in Hellenistic2
a research that was done in free speech of people 15-19 show that they knew better another language (English german italian Francais etc) and use foreign words as loan or turn them to Greek sounds to express their thoughs at 15% !!!!!
the result after school > 18 years is that 75-95% understand Hellenistic and 60-70% Attic
but only 45% can express speech in Hellenistic and 30% at Attic
the differences are among Urban and country(villages)
a research by Stavropoulos in Kallash Pakistan show that even words had changed to another language (example Estia->Tzestia Zeus ->Zeo etc) but sounds remain the same
Kallash although speak today another language their sounds are same with hellenistic and modern Greek, especially s which does not shift to sh often
as you see the more written and the more well teach a language the more stable it is,
the less written the more words change, but sounds resist the change,
you can hear the Γ Σ Δ etc aven d is nt.
Other good example of how languages change is the Slavic language with Cyrillic and Great moravia Slavic, that language kept Slavic languages to a prototype a stereotype so modern Slavic languages are common in between them, although each follows its way, basic Grammar is after the Slavonic of Cyrill,
the Germans
Germanic I believe changed more than Slavic,
Even Luther could not stop it,
But was stabilized at least as deutsch by Goethe
soon the possibilty to became more than 1 languages stoped,
adoption of 1 common agreed form (I think hanover's) stop fast and different mutation process
and push 1 common exelixis,
to understand how fast a language change better turn to USA,
although schools exist a Texan with a NYer have totally different accent,
consider when Texas become a state, and you see
that 3-5 generations is enough,
and if scholl did not exist in Texas surely we might have a new language.