If you choose to install to a non-default directory, then actually during install and whenever you subsequently right-click the drive in My Computer, the error “Could not load language file. Please reinstall the application.” appears.
Investigation with Process Monitor shows it is looking for the following path:
\COMODO\COMMON\LANG
Unfortunately the installation process has put this folder tree under C:\Program Files\ whereas the rest of the install has honoured my custom folder choice.
I had to uninstall and then install to the default location.