Hi Reinhard,
At the moment there will be only one language folder in the lang directory and this is en (english). So the theme_flexi_ii.php you changed was in...
lang/en/theme_flexi_ii.php
What you need to do is add another language folder 'de' (German) into the lang directory and paste a copy of theme_flexi_ii.php in it like so...
lang/de/theme_flexi_ii.php
So in the lang directory you would have...
lang/
en/theme_flexi_ii.php
de/theme_flexi_ii.php
Now you can change the $strings in lang/de/theme_flexi_ii.php to the German equivalent for each string...
Example:
$string['welcome'] = 'Willkommen, {$a}';
and leave the lang/en/theme_flexi_ii.php as it was...
$string['welcome'] = 'Welcome, {$a}'
When you have changed all the string names in lang/de/theme_flexi_ii.php
You need to load the language pack for German (de) if it is not already loaded. Then you need to 'update' the files, this will add to the language files what you have just added, so that the strings get picked for the German language.
You also need to enable the Language filter in Site Administration > Plugins > Manager Filters
Also if you make any changes to your theme files you need to Purge cache...
Site Administration > Developement > Purge all caches
HTH
Mary