Use the content repository and it's method GetLanguageBranches:

IEnumerable<T> GetLanguageBranches<T>(ContentReference contentLink) where T : IContentData;

This work for pages but how can I do that for a block? Because I don't know how to create a ContentReference from a block who inherit from BlockData?

Either you have a block as a property on a parent model, for instance a page. Then you should check the page. If it's a shared block then you should have a reference to that block (either through a property or as part of a property, for instance a content area) and you can then use that reference.

Just to clarify, if it is a shared block instance you can cast the instance to IContent.