globalize >= 4.0.1 already provides uniqueness validation support for translated models via the regular UniquenessValidator. This makes this gem obsolete when using rails 4 - as such it has been discontinued (i.e. the version of the gem in the rails4 branch does absolutely nothing).
Compared to what this gem used to do, globalize just has one restriction (checked with globalize 4.0.2): Uniqueness validation is always scoped by locale. This is what you usually want. If not, you should submit an issue and/or pull request to globalize.
For rails 3.x, please see master branch (versions 0.1.x).