At least in Firefox, it's in the browser settings, under General > Languages > Choose your preferred language for displaying pages > Choose...
This lets you specify an ordered list of whatever languages you prefer. If you haven't set it manually, usually it defaults to whatever your system language is.
It's configurable from within the browser, though it moves around a lot and has been getting hidden lately. I think default is based on the OS language, but of course that can vary between browsers and versions.
This lets you specify an ordered list of whatever languages you prefer. If you haven't set it manually, usually it defaults to whatever your system language is.