A language with no case system and no grammatical genders is still easier
So you prefer Chinese over French?
Seriously, I have the very strong impression that complexity of a language hardly has any negative influence on its proliferation. Look at the Koine in antiquity.
(Although the converse is true: The proliferation of a language has a negative influence on its complexity.)
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Back to the point, assume that the Angevin Empire is unhinged from the French crown - quite possible under several PoDs. That leaves us with two "multi-nation" kingdoms (nominally England and France), both with French as dominant/court language: In "England" there are French, Occitan French, and English people living, most probably it also includes Wales and Brittony; Scottland and Ireland might be included, partially included, or staying outside. "France" has mainly French and Dutch subjects. Toulouse, though nominally French, will be de-facto independent or falling to the Angevin complex.
Now we only need a PoD which makes the major part of "France" join the Western Empire.
To take one of our favourites, let some sort of Burgundian state arise and deprive the French crown of its influence on important areas. With some additional weakening, it is only a question of time when a French king without male heirs agrees to marry his daughter to the "English" king, and make her inherit the kingdom ...
The funny thing is, "English" and "French" will be valid cultural, linguistical, and ethnic terms in this scenario; both terms will have some political content at the beginning, but there is hardly any connection between these two worlds. IOTL, the restriction of the Angevins to the British isles and after that, the 100 Y W led to the connection of ethnic and political notions in this region. Changing this will alone be extremely consequential ...