But now the times have changed "Pali" as a language is dead.
Depends on how you define what "dead" means or, the language is dead to whom. To many Pali scholars like Ven. Bodhi, Ven. Thanissaro, Ven. Analayo, etc. Pali is very much alive and they constantly use it as the source reference to their sutta translation works. Thanks to the effort of many venerable scholars, the suttas have been translated to many different languages in the world. As we speak, there're monks and lay people around the world who are reciting the Tipitaka in Pali, English, French, German, etc. right now.
Is there any mentioning on how beings in different realms understand a single language, I mean even we don't use "Pali" now so how does it happen?
Depends on which realm it is. To the devas in the heavenly worlds, due to the great merits they've generated, many would possess supernatural powers to be able to communicate with humans (Ex: the conversation of the householder Citta with many devas in SN 41.10). If it's the lower realms, then it obviously much tougher due to the tremendous suffering and ignorance throughout these realms. The Buddha in MN 129 used a very famous simile to make the point:
The Buddha: Suppose a man threw into the sea a yoke with one hole in it, and the east wind carried it to the west, and the west wind carried it to the east, and the north wind carried it to the south, and the south wind carried it to the north. Suppose there were a blind turtle that came up once at the end of each century. What do you think, bhikkhus? Would that blind turtle put his neck into that yoke with one hole in it?”
The Bhikkhus:“He might, venerable sir, sometime or other at the end of a long period.”
The Buddha:“Bhikkhus, the blind turtle would sooner put his neck into that yoke with a single hole in it than a fool, once gone to perdition, would take to regain the human state, I say. Why is that? Because there is no practising of the Dhamma there, no practising of what is righteous, no doing of what is wholesome, no performance of merit. There mutual devouring prevails, and the slaughter of the weak.