Wordpress Plugin translation - load_plugin_textdomain
In case someone having the same problem. I had
get_plugin_data( __FILE__ );in my code. This caused to run a wp_core function where it loads the textdomain, so my en_US.mo was overriden by de_DE.mo
I don't know why get_plugin_data() took the wrong lang-file. It seems to have picked the right one for different plugins that use the function.