"Special Price" is not translated

The definitive answer to this question is that Special Price is an attribute.

You can simply translate the Special Price price-label from your Magento Admin Panel, just go to

Catalog > Attributes > Manage Attributes

and open the Special Price attribute (attribute code special_price).

From there select the tab Manage Label / Options.

Here you can set the proper translation for each of your store views.

After saving the attribute don't forget to flush Magento cache.

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:'