In regard to Sturehavns idea I made following steps:
1. Install and activate Loco Translate
2. Inside Loco Translate open the default.po of your theme
3. Make all the translation of strings that you wanne make
4. Save your changes / translations
5. Open your FTP-Client
6. Copy the default.po from themename/lang/default.po to your desktop
7. Rename this file fitting your language
8. Copy this file Inside the same folder, so that you have two versions of the .mo and .po file (default plus your language file)
This will not fix the problem of updating the theme.
Thx one more time to Sturehavn!
