AFAIU, Hindi is one language that uses the Devanagari script. Is this
correct? If that is correct, how about having two lines in HELLO: one
that says "Hindi" and shows a Hindi greeting, and another that says
"Devanagari" and shows a greeting in some other language that uses
Devanagari?
The problem is, like Visuwesh said, most of the languages which use Devanagari use Namaste/Namaskaar as a greeting.
Either way, I will ask for a greeting in Marathi, which uses the
Devanagari script as well.
Marathi also uses Namsakaar.
Of course there are also local greetings, so one solution is to use the standard greetings in Devanagari
and local ones in Hindi, like रामराम (raam raam), पायलागू (paay lagu) etc.