Low cluster is the problem. Your cluster does not respond to the change language command.

You can load new languages with NavCoder. However, it will not change your problem.

With a low cluster, the vehicle will always default to the default language (language 0, or German), because the cluster does not send any language commands.

If you do not wish to change the cluster, then your only solution is to change language 0 (German) to another by editing the text on the nav OS disc, and loading the customised OS.
Which is of course what you wish to do.