Hi Frank, 

Please read our blog post about translation and localization to get a clearer picture of situation. 

If you want just to translate mentioned buttons maybe it would be easiest to just hardcode translation in file 

../wp-content/plugins/wpdatatables/source/class.wpdatatable.php

around line 2016 you can find 

'text' => __('Print','wpdatatables')

just change 'Print' to appropriate text. Similarly you can find text for all other buttons in the code below.

If you want to be able to customize all strings from our plugin you can use approach from that blog post. At some point one of our client translated most of the strings to German language but we keep changing our plugin and that translation is not ready for public release. 

I updated po file from that translation with newest strings and attached it. You can skip most parts in the tutorial just extract zip in languages folder, open .po file with poedit, find strings that you want to translate, translate it and save. If you choose German language in wordpress settings everything, our plugin should be translated too.

Hi Andrey,

We released major redesigned version of the plugin so older translation that we had is no valid any more. Best way to have german translation is to use updated english version and create german version from it following instructions from blog post.