You could just about make the phrase “cultural conservative” do the job, but it has regrettable connotations from American politics.
Then there’s traditionalism, but it doesn’t come prepackaged with the nuance you intend. You’d still have to explain what kind of traditions you mean.
You could avoid an outright label and just talk about “preserving Cantonese identity”. Or feel free to coin a neologism, e.g. “Cantoneist”. There’s no wrong answer in English if you do it with style. IMO.
Edit: Cantonism was already coined for use in Russia, so maybe make your own word up instead to avoid associations with er, pogroms and such. There are no wrong answers, assuming you are being original. Never assume.