I’ve never tried, so the following may not be the correct advice!
I think you first need to download this PO file from here:
https://code.google.com/p/alo-easymail/downloads/detail?name=alo-easymail-de_DE.po&can=2&q=
Then follow instructions here: https://www.appthemes.com/blog/how-to-translate-a-wordpress-theme/ to edit the PO file, and create the MO file.
Then replace the MO file which is in the directory wp-content/plugins/alo-easymail/languages with your new MO file.
