Bergwiesel

It was Safe Mode on :-( So I guess this closed

Do you receive this error when uploading files to other pages too?

Yes, except of the first page (Home).

Do you receive this only if uploading a ZIP, or do you get it when uploading the language pack .json files individually?

Doesn't matter what I upload.

The error message most likely indicates that the directory where PW wants to store language files doesn't exist, and it's unable to create it. You could try creating the directory manually, but I think you will still have issues until you can determine what file system permission is blocking access. If you want to try creating the dir manually, edit your language pack and note the ID in the URL. Now create a dir based on that ID in /site/assets/files/. So if the ID is 1234 then you'd create the directory /site/assets/files/1234/ and make sure it's writable to PW. 

I think it has to be something very basic, since I can't even publish a new page, because of that (missing folder for pictures). I guess I have to start from the beginning...

> Make sure your /site/assets/ directory is writeable.

I did. Everything under /site/assets has 777 and it still doesn't work. There were some error messages due to copying and chmod of the pictures during install, which I can't explain since I made the folder writeable as instructed during the install...

Where should Processwire copy all the json files of the language pack?

I freshly installed Processwire and now I'm trying to build a multilingual website. After uploading a language package and saving I receive the following message:

destinationPath is empty or does not exist (language_files)

I already configured a  uploadTmpDir in my site config.php with no change. Which directory do I have to create or change permissions for?