Need A4 default paper size, not US size

Thanks Matthias for taking the time to answer me. i had already done what you suggested, and it does not affect the default paper size in Pages. i don't really mind adjusting the format from US to A4 each time, so my question is more in relation to the North American users having the opposite problem: how come their default size is A4 when mine is US? Maybe we should all switch computers, or places!
Concerning the themed templates that are off-centered (to the right) after converting them to A4, did anybody encounter the same behavior? i'd love to hear your solution.

iMac 800 G4 flat panel, 80Gb Mac OS X (10.3.8)

OK, i got it figured out, and i'll post the brief solution here for anybody who may stumble on the same 'problem'. i live in Japan, and had my system language preferences ordered as English, Japanese, and French, since we use these three languages on the same mac. Because i wanted my units to be metric and my time display to be in the 24h format, i had created a custom setting in the system preferences.
Fine up to now, except that my Pages must have been disregarding all along my custom settings, and going on its own for the US size (possibly due to my selection of the US dollar as my monetary unit? Just a guess…), so on a whim, i decided to switch back to "Japan" as my region (keeping my custom adjustements), and voilà, when i reopened Pages, the default paper size was now A4. i should add it stays that way on consecutive closings and reopenings, though interestingly, the US size appears first in the inspector window after selecting a theme, and switches to the A4 size after a few seconds (iMac 800).
So i suppose that anybody with a default paper size problem, A4 or US, may check if it is or not a custom setting issue. Finally, all the templates are correctly centered, and not displaced to the right side as they were when manually converting from US to A4. Hope this helps somebody out there!