you've got a couple of options:
a) personally i think that this may be a bug. i.e. the page number shouldn't change when you're drilling into the subreport. if you've got a support contract, you can contact customer assurance...if it is a bug i believe that you don't get charged. however, the odds of getting this fixed in a timely manner are probably small.
b) fast and easy way is to write the translation into the above formula...for example (French speakers please forgive my translation) you can use the contentlocale function...
if contentlocale = 'en_US'
then
(if pagenumber <> 1
then 'Page ' + totext(pagenumber -1, 0,'') + ' of ' + totext(totalpagecount-1,0,'')
else 'Page 1 of 1')
else
if contentlocale = 'fr_FR'
then
(if pagenumber <> 1
then 'Page ' + totext(pagenumber -1, 0,'') + ' de ' + totext(totalpagecount-1,0,'')
else 'Page 1 de 1')