I am not surprised Turkish isn't working. I forgot to mention above that I only put in support for a handful of languages the same as the Amazon plugin does that I copied code from. Unfortunately it is a bit painful for how you assign languages to books in calibre, as every language name has to be mapped to a "code". So any languages I don't have a mapping in place for won't get handled. And since I don't have a list of languages that Goodreads supports (nor the inclination to spend hours trying to divine them and their associated mappings) it is what it is at the moment.
I can add more languages to it on request, but I don't really want to have to do this frequently. So right now it has mappings for:
You have also requested Turkish.
If there are any other languages people want to see supported from the Goodreads site, please post a reply along with a link to an example book on Goodreads that I can test it with. Perhaps in a week or so depending on the rate of replies I will publish an update to the plugin with support for them.