Do browsers localize the OK and Cancel buttons of the javascript confirm popup?
The OK/Cancel button in Internet Explorer will obey the Operating System UI language settings.
The technical reason is that IE is using the MessageBox Win32 API call to display its Javascript dialog box. That API relies on the OS UI language and not the browser UI language, as it is owned by the OS.
As for 3rd party browsers, it really depends if they are using MessageBox or not to display their box. If they are, it will obey to the Operating System language, if not, it will obey to the browser UI language.
EDIT: Firefox and Chrome are not using MessageBox so technically they should be following the browser UI language settings, though I don't have a french install to test it on. CONFIRMED
If you must localize your buttons, your best bet would be a modal DHTML dialog.
My OS language is French and the OK and Cancel buttons are in french so the answer is : yes!
IE :
removed dead ImageShack link
Firefox :
removed dead ImageShack link
Chrome :
removed dead ImageShack link