The translations file Messages_de.properties appears to be in UTF-8 encoding instead of the expected ISO-8859-1 encoding.
This causes the translated strings to contain garbled chars instead of German umlauts. (See attached screenshot for an example.)
I created a patched file by converting it to ISO-8859-1. This file is attached to this issue.
I took a different approach and replaced all non-ASCII characters in the property files for German and Spanish to make the encoding of the file irrelevant and avoid issues with different IDEs/Systems altogether.