This one is related to my previous post. My problem is that after I put messages in ValidationMessages.properties in languages other than English (Ukrainian and Czech in my case) I got messed up characters in the end. The encoding of all those ValidationMessages.properties files in the bundle is UTF-8, the same as I use for the other translations, and those work well.
I don't know what exactly solved the problem, but as I moved from the GWT Validation to the Hibernate Validation libraries it helped.