First, run sudo bash to enter a root bash session. Then execute the following commands:
echo -e "cs_CZ.UTF-8 UTF-8\nen_US.UTF-8 UTF-8\nen_GB.UTF-8 UTF-8" > /etc/locale.gen
locale-gen
That should fix your language settings for now. Reboot and check if your system is still in franken-english or not for me please.