It's actually controlled by the Languages setting in your browser. Both Chrome and Firefox let you set the list of languages sent in the header, along with the order.

Doesn't exactly do a lot of good. Sites tend to ignore it, at least from what I've seen.

The first thing I do on any new installation is peruse the settings and tweak to my liking. It seems I missed that option, being privileged to have English as a mother tongue.

No need for any add-ons or extensions, you can set it the browser directly, although most websites these days ignore it anyway.

The option should be in your OS or browser settings.