What is the name given to the type of assembly that contains localized resources?

You can deploy your application's resources in satellite assemblies. By definition, satellite assemblies only contain resource files. They do not contain any application code. 
In the satellite assembly deployment model, you create an application with one default assembly (which is the main assembly) and several satellite assemblies. 
You should create one satellite assembly for each culture that the application supports. Because the satellite assemblies are not part of the main assembly, you can easily replace or update resources corresponding to a specific culture without replacing the application's main assembly