I could call it Beijingese (or Pekingese) if you prefer. But many people might not know what I was talking about. Mandarin is the common name used in English to refer to this language.
I don’t have any problem if you want to talk about Parisian French (or pick your preferred other name for it), Castilian Spanish, etc.
Parisian French was pushed onto the people within the borders of the French nation-state by force, by a brutal authoritarian monarchy. Quoting Wikipedia,
‘The goals of the Public School System were made especially clear to the French speaking teachers sent to teach students in regions such as Occitania and Brittany; “And remember, Gents: you were given your position in order to kill the Breton language” were instructions given from a French official to teachers in the French department of Finistère (western Brittany).’
The French state continues to repress minority languages inside its borders. See https://en.wikipedia.org/wiki/Language_policy_in_France
