Make sure that you use the exact string for the first quotes. In this case you see this code in the price.phtml:

<?php echo $this->__('Special Price:') ?>

i.e. you need Special Price: for the first quotes in your translate file. So your first try

"Special Price:","Sonderpreis:"

should've worked. In some cases this translation doesn't work. Than you have to force this translation with

"Mage_Catalog::Special Price:","Sonderpreis:"

This goes in your translate file at

app/design/frontend/default/yourdesign/locale/de_DE/translate.csv

Hope this helps!

+++ EDIT after Comment +++

There are 22 occurrences of this 'Special Price:'

22 occurrences http://i60.tinypic.com/ous0nl.png

Make sure that you use the exact string for the first quotes. In this case you see this code in the price.phtml:

<?php echo $this->__('Special Price:') ?>

i.e. you need Special Price: for the first quotes in your translate file. So your first try

"Special Price:","Sonderpreis:"

should've worked. In some cases this translation doesn't work. Than you have to force this translation with

"Mage_Catalog::Special Price:","Sonderpreis:"

This goes in your translate file at

app/design/frontend/default/yourdesign/locale/de_DE/translate.csv

Hope this helps!

+++ EDIT after Comment +++

There are 22 occurrences of this 'Special Price:'

Make sure that you use the exact string for the first quotes. In this case you see this code in the price.phtml:

<?php echo $this->__('Special Price:') ?>

i.e. you need Special Price: for the first quotes in your translate file. So your first try

"Special Price:","Sonderpreis:"

should've worked. In some cases this translation doesn't work. Than you have to force this translation with

"Mage_Catalog::Special Price:","Sonderpreis:"

This goes in your translate file at

app/design/frontend/default/yourdesign/locale/de_DE/translate.csv

Hope this helps!

Make sure that you use the exact string for the first quotes. In this case you see this code in the price.phtml:

<?php echo $this->__('Special Price:') ?>

i.e. you need Special Price: for the first quotes in your translate file. So your first try

"Special Price:","Sonderpreis:"

should've worked. In some cases this translation doesn't work. Than you have to force this translation with

"Mage_Catalog::Special Price:","Sonderpreis:"

This goes in your translate file at

app/design/frontend/default/yourdesign/locale/de_DE/translate.csv

Hope this helps!

+++ EDIT after Comment +++

There are 22 occurrences of this 'Special Price:'

22 occurrences http://i60.tinypic.com/ous0nl.png