I hope this discussion will be continued.

One of the major reasons why we started developing our own plugins was the fact that if we needed complex functionality working in a multilingual enviroment, there was no solution - more complex plugins (for events management, membership) were just not working with WPML or Polylang (which are definitely best multilingual plugins in my opinion).

Providing a core Language API would allow developers to standarize at least some of the methods of handling multilingual content.