OSM is used worldwide so should be multilingual for texts that show on the map (names, for instance) but for database values, English is used (so something tagged with colour=blue will show on the map in blue).
If the hydrant has no name or number, then don't add anything. Many shops in Bulgaria have no name ("Плод и зеленчук" is not a name but a description of what they're selling) so in that case, add no name (but you could add name:signed=no).