It’s a part of the header sent with every internet request. Standard thing to identify the user’s language so you know which version to send

While that’s true, there’s also the “Works on my machine” effect.

It might’ve been less obstructive on their pc than on op’s

The way I interpret the comment is that they are saying “If you would eat normally, you couldn’t be that thin.”

It’s a reference to the fact that energy cannot be destroyed, only transformed into a different form.