The following executed on the console works:

from PyQt4.QtGui import QWidget
class ABC( QWidget ):
	def doit(self):
		print ( self.tr(u'The file "{0}" has been deleted or is not 
accessible'.format(unicode(u'äöü'))) )
a = ABC()
a.doit()

But only if the object (ABC) inherits from QObject, not sure, if in 
your case that's true...