I know the present perfect tense may not be used with temporal expressions which define a timeframe which does not include the present. For example:
WRONG: I've visited him on Monday.
However, when reading The Cambridge Encyclopedia of the English Language, this is what I have found:
The final section looks at what has happened to the English language in the 20th century, and in particular at its increasing presence worldwide.
I understand that grammar rules have their exceptions, but I can't wrap my head around why David Crystal used the present perfect instead of the past simple. It can't be a mistake, can it?