The "Target Locale" setting under the Asset field's settings is referring to which locale do you want to select assets in. When it's set to "Same as source", then, for example, if you're viewing an entry in German, the Asset selector modal will only display Assets assigned to the German locale. And if you were viewing the entry in Dutch, it would only show Assets assigned to the Dutch locale.
If you change the "Target Locale" from "Same as Source" to a specific local (i.e. German), then no matter what local you're viewing an entry in, only Assets assigned to the German locale would show.
When saving a new entry in locale 1, Craft "copies" content data (anything stored in the
craft_content table in the database) over to the other locales for fields marked as translatable as a starting point for the other locales. All relational fields (Assets, Entries, Users, etc.) aren't "content" and they are stored in the
craft_relations table and Craft currently doesn't copy that information over.
Ironically, if you uncheck the "This field is translatable?" checkbox in the Asset field settings, it would show up on the other locales, because in that context the Assets aren't stored on a per-locale basis.