I'm not aware of any built in support to the .NET Xml comment mechanism to support localization in the source comments - and that may be because it's going to get pretty unwieldly in the source pretty quickly.
Document! X however does directly support localization of your documentation using Content Files - allowing you to work, track and verify your localization to any number of languages within a single documentation project. This support extends to project properties, conceptual topics etc. too so you can handle the whole project localization.
There is a demonstration movie available that illustrates how the localization features in Document! X work with a .NET project:
There are additional localization helper features in HelpStudio for tracking and importing/exporting for localization by third parties - those features are available in Document! X and HelpStudio if you have the bundle of both products installed.