Hi emninger,
No, there is no cheatcode for timezone settings AFAIK, as well as no CEST timezone configuration in the default SliTaz.
As you already installed "locale-de" package, then please follow the path:
Applications menu -> Settings -> Time Zone configuration
Choose country (Germany) and click [->] button
Choose town/city/area: Berlin or Busingen? and click OK
That's all.
Log out and log in to be sure settings are applied. Now SliTaz will switch Daylight savings time automatically for you.
Now maybe you need to set up your clocks because TZ settings are changed.
Simply open this link: TazPanel: Settings (you need to enter "root" and it's password to come in). There you can click the button "Sync online" to synchronize your system clocks with the time server tick.greyware.com precize clocks.