Maybe use a dash ('-') for unknown speed and duration, instead of 0?

you don't have to find a single notation for all languages: use _(...) to let people translate...
Something like that, (I think it needs the if else so that i18n tools pick the string to localize)?

if task.speed > 0:
   status_text  = _('%(status)s (%(progress).0f%%, %(rate)b/s, %(remaining)s)')
else:
   status_text = _('%(status)s (%(progress).0f%%, %(rate)s/s, ?s)')
status_message = status_text % {
                   'status': task.STATUS_MESSAGE[task.status],
                   'progress': task.progress * 100,
                   'rate': util.format_filesize(task.speed))
                   'remaining': util.format_time(round((task.total_size * (1 - task.progress)) / task.speed)) if task.speed > 0 else 0)
}

Maybe use a dash ('-') for unknown speed and duration, instead of 0?

Oh ya, I forgot about dash.

status_text = _('%(status)s (%(progress).0f%%, %(rate)b/s, %(remaining)s)')

Did you mean %(rate)s/s with an s instead of b?

status_text = _('%(status)s (%(progress).0f%%, %(rate)s/s, ?s)')

Is the ?s meant to just be ??

And do you think we should make these translatable or just use dash? It would allow the /s to be translated, except that format_filesize() doesn't use translatable strings for the size units. Should we also make those translatable?

status_text = _('%(status)s (%(progress).0f%%, %(rate)b/s, %(remaining)s)')

Did you mean %(rate)s/s with an s instead of b?

no, b/s, my mistake

And do you think we should make these translatable or just use dash? It would allow the /s to be translated, except that format_filesize() doesn't use translatable strings for the size units. Should we also make those translatable?

yeah I think they should be translatable, also format_filesize(), having a look at various translations of https://en.wikipedia.org/wiki/Bit_rate. You could ask on the #518 discussion what makes sense to our translators.