It's the same language. I'm a Spaniard, so I know it well. Name it the way you'd like, it can be called Spanish, Español or Castellano everywhere from Mexico to Patagonia, and from The Canaries up to the Pyrenees.
"name it the way you'd like, it can be called Spanish" is a very different proposition to "[you should say] Spanish, Castilian does not exist [and you are wrong to use that name]", which was the angle of the poster who kicked all this off.
> from Mexico to Patagonia, and from The Canaries up to the Pyrenees.
Sounds a bit imperialistic?
Notwithstanding the tens of millions of native speakers of autochtone non-spanish languages in these territories: Mapuche (260K), Quechua (7.2M), Aymara (1.7M), Guaraní (6.1M), Wayuu (400K), Mayan (6M), Miskito (150K), Garifuna (120K), Nahuatl (1.7M), Mixtec (530K), Catalan (4.1M), Basque (750K), Galician (2.4M). Spanish is quickly eroding all of these, but they still exist! (And this only counts native speakers. The number of people who are fluent in Guarani or Catalan is certainly more than the double of that.)
> Not imperialistic. Would you say the same of the English language too?
Yes, of course? When I think of imperialism the first thing that comes to my mind is precisely "the bri'ish empi'ah and its commonwealth"! If English is currently the world's default language is just because of the triumph of English/American imperialism.
