You do know that yugoslavia too "united the slavs" , this is not the answer and he isn't responsible for the unity in the longer run.

It was because Yugoslav states had more autonomy and was dominated centrally by the serbs, while india despite being a union was very centralised and still is very centralised in many aspects, india is gradually going from central to federal slowly but surely

While one can say that india is also dominated by the hindi speaker, india still had decent central governments which backtracked on decisions which affected their sovereignty at a local level, other than which they also didn't hold back while using military attacks, which worked in the longer runs - the naga, mizo, naxals etc etc since unlike yugoslavia which barely had the diversity india had a good amounts of communities which didn't care about a certain issue while also having communities which did

For example, literally no farmer protested outside of punjab-haryana yet the farmers protest was one of the biggest protest

So contrary to the post, having many communities and those communities having different shared goals actually helped india in terms of staying together, obviously it comes with many issues like struggling to make decisions without offending a community which causes inefficiency, but hey I'll take inefficiency over getting killed and going balkan mode.