Other useful metrics (*much* harder to gather) are:
- Number of general enrolment courses advertised for a specific period. (It would be better to get information on whether the course was successful, and how many attendees there were, but I don't think training houses are publishing that information ;) ). Also might consider university subjects covering that language.
- Number of "high quality" books being sold/written for that language. Not necessarily just O'Reilly books, as other publishing houses do make good books too, but let's not include the dodgy ones.
Number of (active) language-specific user groups. (Active might have something to do with busy-ness of mailing list and regularity of meetings, or perhaps one or the other).
Number of language-specific conferences, or perhaps number of representative talks at "programming" conferences and have a wide selection. Could also consider size of attendance.
It'd be great if it could be done. ;)