Geany itself does not constrain languages used for plugins, any language that can present a C call compatible interface and which can call C can be used. For example I once used Cython successfully (a long time ago, no guarantees it will still work, but no theoretical reasons why not).
For languages that are interpreted (eg Python or Ruby) there is a third party plugin that supports plugins in those languages, see https://github.com/kugel-/peasy.