will parrot be a universal plug-in language?

It's possible, and it's still very much part of the plan. Parrot's current development focus is in making Rakudo Star successful, but one of Rakudo's long term goals is interoperability with Perl 5. To make that work, Parrot has to support enough interoperability that it's reasonable to support other dynamic languages.

This means, of course, that other languages need to be able to target Parrot, but there are several such implementations in progress now (and we've demonstrated interoperability between languages several times).

Parrot's embedding/extension interface hasn't seen much work, and it needs attention, but it hasn't been a priority lately because we haven't made it a priority. With that said, the Perl 5 interoperability has begun again, and a newer project is making a PL/Parrot interface for PostgreSQL, so that system will see more attention in the near future.