my current workaround looks like this:
every plugin has it's own xliff message foreach module.
in this message file, there are all interface translations which are
unique to the corresponding module.
furthermore, there is a global xliff message file containing common
strings, which are used throughout the whole application.
it's located in plugins/ullCorePlugin/i18n/common.de.xml
to bad, that symfony doesn't check in this path for translations, so
there is a symlink from apps/myApp/i18n/ to this file.
if you want to use a translation from the common messages you have to
use __('MyString', null, 'common');
if this information is not helpful enough / on spot, let me know!