I have a multi-site setup and need to change the default language on one of the sites.
Is there a straight forward way to do this in Episerver, without effecting the other sites' configuration?
Yes, You can do it using the manage website section in the admin panel.
This is set in admin mode > Config > Manage Websites > [select site] > Hit edit button
From the dropdown list pick the culture. This will set the default mapping for the domain
it depends either you need to change language detected by incoming request/host or you need to change language for content (aka master language).