Hi Taka,
・The role of English has developed to such an extent, unprecedented in the world history, that it is difficult to see how it can now be displaced.
If 'unprecedented in the world history' were placed in front of the sentence as:
・Unprecedented in the world history, the role of English has developed to such an extent that it is difficult to see how it can now be displaced.
would it still make the same sense? Or it's basically:
・The role of English has developed to such an extent, which is unprecedented in the world history, that it is difficult to see how it can now be displaced.
so such placing is no good? In the original, it modifies 'extent'. Move it if you want it to modify 'role'.
To me, it makes more sense to say that a role is unprecedented than that an extent is unprecedented.. However, whichever word is modified, the wriiter needs to explain what he means by 'unprecedented'. eg Is he talking about number of speakers? Use by a large number of countries? Use as the language of the scientific community? Rapidity of adaption by non-native speakers?
Bbest wishes, Clive