Awesome, quick question Rated 5 out of 5 stars

Works great but question is why are there two values? For example en-US, en?

Great when it works Rated 4 out of 5 stars

It occasionally disappears from my toolbar, and the customize menu can't bring it back. Not sure what triggers it to come back again.

simple and useful! Rated 5 out of 5 stars

...but, as a lazy user, living in 2015, it would be nice to have a selection pulldown to choose from.
Maybe some user editable pre selections, I can choose from...

Rated 4 out of 5 stars

Works great, but we need a way to get back to the original value

This review is for a previous version of the add-on (initial.rev102.1-signed). 

Done! In 0.9.1, if you erase the value in the entry box and hit return, it will reset your Accept-Languages header to your locale's default.

Does exactly what you'd want! Rated 5 out of 5 stars

Does what it says, but a few small tweaks would greatly improve the user experience.

The ability to reset the value back to what it was before I typed something in would be nice. Also an option to refresh the page after changing the value would be hugely helpful!

This review is for a previous version of the add-on (initial.rev102.1-signed). 

Thanks for the ideas -- I didn't realize people other than myself were relying on this :) I'll definitely put some time into the UI/UX soon.