What happens if Always On High Availability nodes have different connection default languages?
If the application is setting the language on the session when it logs in, then the default language doesn't matter.  If it's not specifically setting the language on the session, then there is risk of date strings being converted incorrectly, and dates being returned in American English format instead of British English.

Unless there is some unstated reason, I recommend simply changing the default language on the login on the secondary to match the primary.