I create epubs which contain different languages so need the language correctly attributed on xhtml files after exporting –

selecting different languages in different paragraph styles affects typographer's (curly) quotes/straight quotes – I'd expect the to follow the app preferences setting –

Advanced Character Formats > Language > [No Language] = gives straight quotes

Advanced Character Formats > Language > English: UK = gives typographer's quotes (curly)

Advanced Character Formats > Language > cy_GB = Welsh = gives straight quotes

InDesign 17.1 – Mac OS 12.1 – iMac (Retina 4k, 2019)