I just discovered this plugin, and I'm thrilled to see this active discussion on getting it WPML-compatible.
I'm not sure what all this might involve, but a multilingual version might be more involved. Here are a few other needs I can think of (these are real use cases in my website, not hypothetical ones):
* The e-mail notification templates would need a version of each template in each language. The WPML String Translation module can handle translations of some plugin strings, but I don't know if it could handle Subscribe2's template keywords. Perhaps Mercedes could shed light on this.
* Some users want multilingual notifications--that is, they want to receive posts from multiple languages, not just one language. Whereas it might make sense to use the current site language for public subscribers, registered subscribers should be given the choice of languages. Even in the digest setting, registered subscribers should be given choice of languages to receive. Thus, I suggest to revise Mercedes' suggestion above: rather than storing just one language code per subscriber, multiple codes should be stored. However, the subscriber should pick one language as the default (for receiving e-mails).
I know this is a lot to ask from a free plugin. Regardless of whether you can implement this multilingual functionality, thanks for providing such a great plugin.