There’s no way to do what you want as far as I know. The iPod classic never supported Hindi or any other complex script, and there is no way to add anything. See the tech specs
Just update your iPhone using your iPhone update function, not the one offered in iTunes.
I have overseen another possible solution in the link of the question:
"Finally just put iPhone into Recovery Mode and did an update.
SUCCESS. It did the 12.4.1 update and all seems fine."
Probably that would have helped as well. But the easiest solution is ...