There's an excellent article on FIM 2010 R2 SSPR customization at this location
including how to customize string settings, together with a table to items that can be customized.

One of the challenges of working with the strings.resources file (or any XML file for that matter) is the ease of accidentally violating the XML file format. Even the best of us make these mistakes. Even with an XML-aware editor, mistakes are common.

The file format used for SSPR customization is the Microsoft XML Resource File format that Visual Studio has a handy editor for.

Here's how to work with these files:

Open the Visual Studio IDE,
From the File menu, select 'New' and 'file'

Expand 'web', expand either 'Visual basic' or 'C#' and select 'resource file' and click 'Open'

Next -
Enter name and value pairs for items you want to customize.
Use the table in the article above to find the names of the customizable UI elements. You can add an optional description that doesn't appear in the UI.

Once you have entered all the customized strings, save the file and then change the file name to strings.resources (VS will save the file with a default extension of .resx)

You now have a SSPR customization file without having to edit the XML directly.