FastDateFormat apparently doesn't respect the locale it was sent on creation when outputting week in year (e.g. "ww") in format(). It seems to use the settings of the system locale for firstDayOfWeek and minimalDaysInFirstWeek, which (depending on the year) may result in the incorrect week number being output.
Here is a simple test program to demonstrate the problem by comparing with SimpleDateFormat, which gets the week number right:
If sv/SE is passed to Locale.setDefault() instead of en/US, both FastDateFormat and SimpleDateFormat output the correct week number.