Scout uses the java.util.ResourceBundle mechanism for native language support. So whatever language files you have in your '<project-prefix>.shared/resources/texts/*.properties' are taken as translation base.
If your application starts with the -vmargs -Duser.language=fr or eclipse.exe -nl=fr the translations in Texts_fr.properties are considered. In case of any other user language the translations in Texts.properties are considered.
In your case delete the Texts_de.properties file and use Texts.properties for German translations. Aware to face all users with an unknown user language with German texts;).
The only confusing thing is the label default (English) label in the text dialog.
So whatever language files you have in your '<project-prefix>.shared/resources/texts/*.properties' are taken as translation base.
The directory where the translation files are located is defined by the Text Provider Service. In the default application template (created by the SDK) the DefaultTextProviderService uses the location you have given: