The way that 'ALL' is in all caps implies that the effect works on ALL cities of ALL civs, otherwise it would not be worth putting in all caps like that. But, in that case, it could easily just state the fact it works on includes cities you don't own. Though, it that is not the intent and this just works on your own cities, then please remove the all caps on 'ALL' and have it just state it works on 'all cities you own' instead of leaving it vague, so new players like me don't overthink it.