I think the only Indian languages in Devanagri script are Hindi and Marathi. Others, like Gujarati, Bengali etc are modified versions of Devanagri.
A Hindi speaking person who cannot understand a word of Marathi will still be able to read 98% of the language and pronounce it correctly, but he will not understand it. Vice versa.
It's similar to how an english speaking person will be able to read German or French, since they're all in Roman script, but will not understand it...(nor pronounce it correctly, since the Roman script is not phonetic).