If languages are defined by standards, what makes them open source? Well, the specification itself can be proprietary. You'll find in older main frame languages if you wanted the specifications you had to pay huge amounts of money for the detailed specifications.

The second opportunity is the implementation itself. For example with Ruby, there are several implementations, all of them happen to be open source.

But the last aspect is the culture. Some languages lend themselves to open source. If you look at the evolution of SmallTalk, you'll find that it didn't really reach the highest levels of utilization until it was implemented as Squeak.