One of the the new features that has been implemented for 0.5 (release date yet to be determined) is support for Translations. For Sigil's first supported language Grzegorz Wolszczak has provided a Polish translation. Currently translations are loaded based upon the current system locale. There no support for choosing the language via preferences. This may come at a later time but for now I believe that using the system locale will handle the majority of user needs.
I've put together a wiki page with instructions for creating translations. This first revision is a bit basic but as people have questions I plan to update it to make it more robust.