Resource Manager is not using resx files?

I created a language pack for the Resource Manager (de-AT). For some sites it works, for some not.

I tried to figure out where the reason is, uninstalled the package and reinstalled it - still the English labels. I double-checked the file permissions and the entries in the database. I found no errors or warnings in the developer tools console. Finally, I changed the line in ResourceManager.resx (not ResourceManager.de-at.resx!) from

<data name="AddFolder.Text" xml:space="preserve">
  <value>Add Folder</value>
</data>

to

<data name="AddFolder.Text" xml:space="preserve">
  <value>Blah!</value>
</data>

After a restart of the application pool, clearing the cache (on the server and in the browser) and reloading the page, it still shows "Add folder".

The stupid thing is: I created this language pack for DNN 9.11.0, and after upgrading to DNN 9.11.1 it has this behaviour. It worked with DNN 9.11.0. And - as I mentioned - this is not the same for all (9.11.1) sites, on some it works, on some not.

Any ideas?

Happy DNNing!
Michael