Currently un-localizing an item loses all traces of the content. Tridion doesn't retain any item version history for localized items when they get un-localized.
The question is: what is the problem you want to solve?
Note that these problems may have different solutions, so it is good to know which problem is the most common and urgent to provide a solution for.
This is closely related to the fact that many different entities in Tridion don't have versioning or any way to audit their history.
This is a legitimate use case for large enterprise customers... a large Financial customer in the US required this, and we had to build a completely custom implementation that used Event System to log out to external ORM system because key things are not versioned. In addition, we had to design their solution to use custom components with custom schema to hold data so that it would be versioned, while it would have been much easier and cleaner design to use metadata of Publications/Folders instead if that was versioned (or at least auditable).
And, localized content was a particular issue for their implementation, so the customized implementation above had to automate the localization of content via the publishing process, and security was locked down so that no users were able to touch content in the publications where content was localized.
We've developed an extension that forces users to enter a 'reason' for unlocalization - having something out of the box would be much nicer but I'd be very interested to discuss what the user/audit experience is of that too.