When I first installed elementary, I had the Intel HD5500 glitch and accidentally made my root language Danish. While I managed to change my user language to English, I can't seem to figure out how to get it changed for root uses (when I open Files as administrator for example).
Make sure your system is up-to-date first (there have been updates related to language settings recently):
sudo apt-get update && sudo apt-get upgrade
Then go to System Settings > Language & Region (example screenshot), click on English in the left sidebar and then "Set System Language". You will likely have to enter your password to proceed.