Weird behavior of XCode4.6 base localization: Works on simulator but only PARTLY on device

Weird behavior of XCode4.6 base localization: Works on simulator but only
PARTLY on device

I tried to use base localization for a project using XCode4.6. The weird
thing is that it works completely on the simulator and on an ad-hoc
testing device that loaded an archive by iTunes, but on the development
iPhone connected to XCode, with language setting to German, only the
Localizable.strings are shown in German, but none of the
MainStoryboard_iPhone.strings - they are all shown in English.
I have cleaned my product, deleted the app completely from the development
iPhone, restarted Xcode etc. The result is always the same: The
development iPhone does not translate the storyboard files, although they
are translated on the simulator and on an ad-hoc testing device.
My configuration: I have a base localization for the storyboard, and a
localization of it in two languages (English and German), i.e. in my
project folder there is a folder Base.lproj containing
MainStoryboard_iPhone.storyboard, an en.lproj folder containing
InfoPlist.strings, Localizable.strings, and MainStoryboard_iPhone.strings
(apart from MainStoryboard_iPad.storyboard, which I don't use right now),
and a de.lproj folder containing files with the same name.
My base localization is English. So the English
MainStoryboard_iPhone.strings file contains content like
/* Class = "IBUIViewController"; title = "Help"; ObjectID = "1Yu-La-mu2"; */
"1Yu-La-mu2.title" = "Help";
and the German version
/* Class = "IBUIViewController"; title = "Help"; ObjectID = "1Yu-La-mu2"; */
"1Yu-La-mu2.title" = "Hilfe";
The Localizable.strings files contain the usual like "STORE" = "Store"; in
the English version, and "STORE" = "Speichern"; in the German version.
I run out of ideas, and could not find anything related on the web. I
would be glad if anybody else had an advice what to do.