If you're having a german JVM for example the resource bundle resolving mechanism does not resolve english correctly.
PeriodFormatter periodFormatter = PeriodFormat.wordBased(Locale.ENGLISH);
String periodAsString = periodFormatter.print(new Period(1, 1, 1, 0));
In this case the german output is returned instead of the english one, because the resourcebundle messages.properties is not correctly resolved. Resolving the French locale from messages_fr.properties for example works as expected.