Hi,
just in case any plugin authors are interested in adding internationalisation to their plugins, it's the same way as for usual applications. So, if you are using autotools, add a po directory to your source tree, adjust your Makefile.am to use it, add gettext macros to configure.in (or configure.ac) and in your code, initialise the gettext system in a similar way as it is done in Geany's src/main.c locale_init().
If you include Geany's support.h, you already got the _() and N_() macros defined, otherwise you can include <glib/gi18n.h> which also provides these macros.
If you have any questions, just ask me.
And feel free to ask on geany-i18n@uvena.de for translations for your plugin ;-).
Regards, Enrico