I would like to say it is unfortunate.
Every body who migrates to other place should adopt to local culture, should respect local culture
( adopting to local culture does not mean leaving ur own culture or insulting ur language or something )
this goes for everybody not only north indians but for all people spread all over the globe.
If u migrate to place leave there as u can earn ur bread and butter, people did accept u for more than years to come as if u were one of them, I consider it is ur duty to respect their culture.
same goes for Indians who have settled abroad. If u don't respect local culture and try to impose ur culture on the locals the unfortunate incidences as occurred in Australia are bound to happen.
So open ur eyes, rectify ur self.
coming to point issue of less use of Marathi in Mumbai,
Tell me one thing how many people now send their children to English medium schools rather than schools which teach in local Languages, I agree learning english is need for time but it should be preferred as second or third language,
As per me Local language should be first language, Hindi should be second language, and english should be third language.( preference for first and second language can be mutually interchanged )
as per me adopting cosmopoliton culture does not mean that letting ur own culture threatened time and again.