Unit test for character encodings in property files

[solth]

Is your feature request related to a problem? Please describe.
The character encoding of property files like messages_de.properties and messages_es.properties containing the German and Spanish translations is sometimes changed accidentally in the repository. This leads to special characters not being displayed correctly with the assumed character encoding of UTF-8.

Describe the solution you'd like
Taking a cue from #6293 (comment) a unit test should be added that automatically checks whether the message files are commited with the correct encoding. This could be done by decoding messages with known special characters from the property files with UTF-8 and checking whether the result is displayed correctly, matches prepared string containing these special characters. Since the test would need to check the property files not from test/resources, but from the main/resources directory, this should probably be added to test ressources, as described here.

Additional context
The problem with the wrong character encodings is described in #6324