Oh, I see !
As I was not finding translation file, I was thinking the translation was hardcoded in the binary files.
Using .qm files, I've already successfully translated the system time dialog box.
The translation keys are actually the English translation. For finding them, I looked at the binary executable file (with khexedit) and at Qtopia 1.6 free sources (not the same version than the zaurus, but it has probably not changed a lot). I take the .ts file from Qtopia 1.6 and translate it, and then generate a .qm file with "lrelease" and put it into /opt/Qtopia/i18n/fr/systemtime.qm . And it works !
Thank you !