The source code (UploadAction.java) contains non localized code.
I am currently logged in French and see this error reported.
Nothing in this page is properly localized.
Why there is an error is another issue.

The source of this class contains other hard coded strings which seems to be displayed to the user in some other circumstances.
Test these ones in a foreign language too.

• Update, I have found the issue, a missing parse of content-navigation.vm containing the require pageid parameter *

This being said the localization requires work (see images)

• drag and drop placeholder
• temporary window
• error window
• non supported file (boot.ini)