I think it is very petty and unfriendly to stand by the two-language name. No one in Poland would ever even know that few meters of the dam are in Poland or would ever suggest that it is a polish reservoir. When you look at the map and find the polish name deep in Slovakia, basically any reasonable person would tell you there is no reason for it. If you really want to, I see no problem in splitting it and letting you name the small part in Polish. Maybe the general rule says to have bilingual name, but the actual situation on site and in anyone's mind dictates that bilingual name here is unreasonable. And that should be above the general rules. 