You're both right. The language parameter is dealt with by means of System Preferences: Language and Region. That's because Scrivener, in spite of its formatting features, is not a text processor.
Yes, the only way to check the spelling in different languages is by changing the language of Edit: Spelling and Grammar: Show Spelling and Grammar every time (shortcut: Caps Cmd dot, or Cmd colon).
Handling the language parameter in paragraph and character styles would make Scrivener more powerful but less versatile. I'm not a coder, but I've seen how Pages 5 has lost that functionality. That's true, Pages 5, in order to become more compatible with iOS, doesn't store the language informations in the styles anymore. To me, Pages 5 is almost useless because of this.
And then there is the matter of operating systems. We get to configure the language of the interface, and that parameter is what is used by many applications: Byword, Multimarkdown Composer, TextEdit, Textastic, TextWrangler, Mou, Sublime Text… I use the shortcut 'Cmd colon' a lot.