Well, that feature is now officially available to everyone!
hreflang is the attribute to communicate to Google if there are alternative language based web pages for the specific web page the attribute is placed on. This tool helps you debug the most common issues around implementation of this technique.
(1) Missing return links: annotations must be confirmed from the pages they are pointing to. If page A links to page B, page B must link back to page A, otherwise the annotations may not be interpreted correctly. For each error of this kind Google may report where and when Google detected them, as well as where the return link is expected to be.
(2) Incorrect hreflang values: The value of the hreflang attribute must either be a language code in ISO 639-1 format such as "es", or a combination of language and country code such as "es-AR", where the country code is in ISO 3166-1 Alpha 2 format. In case Google indexing systems detect language or country codes that are not in these formats, Google will provide example URLs to help you fix them.
Google also moved the geographic targeting setting to this part of Webmaster Tools.
Forum discussion at Google+.