Why can´t I change the language in the webmailer?

If you change the language in the webmailer and the change does not apply, make sure that you have the needed locales installed. You can do this and install new ones with the following command:

 
 # dpkg-reconfigure locales
 

Select your needed locales and generate them.

If you want to use German, be sure that you have selected de_DE and not de_CH or de_AT. UTF8 is not enough, you need also 'ISO-8859-1' on your system.