"We will leave our town tomorrow."
Translation:Tutatoka mjini kwetu kesho.
-ondoka and -toka almost always take a location phrase rather than an object.
You could say Tutaondoka
katika mji wetu ... or using another word instead of katika (such as kwenye or ndani ya) but I don't think it's really correct to just have mji wetu without turning it into a locational phrase.