In 2006 I reported a bug in Xoops that there is a real problem in date notations in languages other then English. The bug was confirmed by Marcan and Mith, but Skalpa never had the time to review/solve the problem.
Short explenation:
When dates are called like: Wednesday 12th of May, you can't let Xoops translate it into Dutch for example (it would be displayed/renamed to "woensdag 12 mei"). We have tested it with other languages/server configurations etc, but it seemed the main problem lies in the Xoops core.