I was looking for good practices on the wiki, on https://wiki.openstreetmap.org/wiki/Multilingual_names#Shared_boundary_features (about objects on country boundaries) there it says:

set name=* to both names, separated by a forward slash with spaces in between

For the Rhine river, this would be:
    name=Le Rhin / Rhein
    name:fr=Le Rhin
    name:de=Rhein

So I would suggest "Oravská priehrada / zbiornik Orawski"