FYI the latest version uses https://github.com/fge/msg-simple which uses threads in a way that is disallowed on GAE. I don't have time to adapt the code to use appengine's ThreadManager, but the offending line in the stacktrace, in case you're interested, is
com.github.fge.msgsimple.provider.LoadingMessageSourceProvider$1.newThread(LoadingMessageSourceProvider.java:91)