My experience was that you couldn't take a random C# project and run it on Mono and expect it to work. I don't want to diminish the technical effort that went into Mono, but it would be misleading to say that open-source C# worked without further qualifications.
I don't think it is. What does it mean for a language to be open-source, if not that there are one or more open-source implementations of that language which support most or all of the extant ecosystem for that language.
Top 5 languages were Java, C, C++, PHP, Python. All five of them were open source in mainstream implementations (Open JDK, GCC, GCC, PHP mainline, Python mainline).
