The problem seems to be that the logic in our StringFormatConverter, DateTimeFormatPatternConverter, and DayOfWeekFormatPatternConverter value converters use the CultureInfo instance that is passed into them. That seems to always be en-US, even when you change the thread's current culture. We're updating all those for the next version (likely 2016.1) to use CultureInfo.CurrentCulture instead. That gets the calendar itself fully rendering properly in your scenario.
The "Today" button text on the other hand is a string resource in our Editors assembly that you can alter as appropriate for other languages, since it only ships with English. Please see our documentation on string resources for info on modifying them.