I changed my admin locale to de_DE and installed a language pack for de_DE which is used by Magento without any problems - for frontend and backend.
But it seems that my custom module's translation CSV (vendor/xx/xx/i18n/de_DE.csv) is not used for the module's adminhtml/system.xml strings, even though all options have the attribute
translate="label comment". The CSV works perfectly for frontend.
If I copy my strings into the language pack's de_DE.csv everything works in the backend.
Does anyone have a fix for this or is that a known bug?