Hi Gianluca,
Thanks very much for the helpful response. I agree with your recommendation, linking building across authoritative sites for each of the languages targeted is probably the best way to approach this.
Interestingly, Google Analytics indicates that the vast majority of French (language) visitors are indeed from France, with less than 6% coming from other french speaking countries, so I think it makes sense to register a country top level domain in future to ensure the best chance at competitive visibility here (and in the UK, US).
What you've written about Yandex is very interesting. I myself am new to global SEO so will be doing research. I will also email seomoz to see exactly how best to handle the subfolders.
Thanks again!