As far as I know there's no general way to do this for an arbitrary config variable, but in your case, for hyphenator in particular, you have a couple of options:
- Convince the author of the Sphinx hyphenator extension to add support for a directive that sets the panguage per page.
- Since the hyphenator extension uses hyphenator.js, which is a JS library, you could probably use some raw html to inject a script tag that would set the language on that page.
- Create a custom template (or extend your existing one) to contain the logic which sets the language for the page.