The below applies to the Openthinclient Pales appliance.
The appliance (virtual machine) is supplied configured for German. Using the MATE terminal on the desktop, first, edit ~/.dmrc and change the language from de_DE.utf8 to en_GB.utf8 (or en_US.utf8 as appropriate).
Then, export LANG=en_GB.UTF-8
and finally use sudo dpkg-reconfigure locales to change the installed language from German to English (deselect German and select the appropriate vesion of English).
Reboot and you should now get English for both console and desktop.