In the last constitution all languages spoken in Nepal were recognized as national languages, if my memory serves me right. Nepali was the official language to be used in courts and government offices.

Jha was making a political statement by taking the oath of office in Hindi. As someone who worked in government in the past, he was well versed with Nepali but he chose to take it in Hindi to make assert Madesi rights and make a statement to his party's constituents. Whether that was a good move or not is for history to judge but I think he started on the wrong foot with that move and created a Pahade backlash of sorts.

Going forward there will have to be some sort of consensus on what language will be used in government. Taking an oath has more symbolic meaning than lets say registering your land in Hindi or filing an application in Hindi at the police station. I personally feel it is unlikely that Hindi, English  or any other language can replace Nepali as the official language in which the central government conducts it's business since the logistical cost of doing that will be enormous. That said, I can see a scenario where provincial governments could use other languages. What I dont undertsand is why would anyone want to fill a form in Hindi, Bhojpuri, Maithili or even Newari , Rai etc since most people who are literate are so in either Nepali or in some cases English. As it is government work in Nepal is unnecessarily complicated, why add more chaos and confusion by adding more languages to deal with ?