What if England had been divided just at one point along regional lines to different sons?
You'd need England to be divided politically after the conquest then : at the exception of Welsh Marches or Lancaster, it always was concieved as a whole entity, when Anjou or Aquitaine weren't, historically, "legally", customary, etc.
You could change it with the right PoD (probably in the late Xth) but giving the narrow conditions of Norman conquest, it could butterfly it really easily.
The closest thing I could see would be having William dying in England shortly after Hastings, and being replaced in a blurry way by his sons with a greater feudal take on England (Odon de Bayeux forming its principality in Kent, for instance)
Eventually, it could be enough for that a king may have to use an
apanage-like feature (admittedly it was the case for Lancaster IOTL) in order to control more efficiently a more divided England.
I stress that it would be certainly more limited (would it be only for demographical reasons) than what existed in France or Germany.
Would it see a radically different country today?
Closer to contemporary France maybe, but the butterflies would be too huge to really have a take on "today".
Lesser parlementarian tradition, English king seen as an arbitle of public order and political balance, and *if* continental holdings in addition of Normandy ITTL, a more politically unstable Norman/Angevin ensemble (giving that it was pretty unstable historically, it's going to be "interesting".)
Would England, and in extension the rest of the British Isles have been more decentralised as a result in terms of culture and institutions?
Hard to say : more regional particularism and historical tradition is a given, but historical decentralization doesn't hold anything sure for 1000 years onwards (compare feudal France to modern and contemporary France in matter of decentralization for instance).
Would it,in the critical stage of development for the English language, have resulted in the emergence of several english languages eventually more disparate than their Anglo-Saxon mother dialects?
Probably not. Not enough population, or exclusive geographical borders to ensure the appearance of ansbau language (to say nothing of their maintain).
Would it also mean more war within England and a stronger Scotland,Wales and Ireland?
It's possible that a part of English ressources would be diverted to inner infighting.
It doesn't mean that Wales, Ireland or Scotland would have it better : these were pretty much divided themselves (the celtic high-kingship being the rule, up to Davidian Revolution in Scotlant)