I've never heard anyone in Nepal saying Ratna Udyan, Mitra Bhag, Sungkhey Udyan. "Park" is now as Nepali a word as English and most of the words you mentioned above are as well. If you are translating for Nepal government or for some official documents, that's a different situation; otherwise, the use of the above "borrowed" words is just fine. What matters is who are you doing it for. If you are doing it for those Nepalis who can't read English, I don't see the need of rupantrit words.