I apologize for the late reply.
I have tested a number of versions of the DLLs but I was unable to reproduce the error.
By the version of the DLLs you are using I am assuming you are using Silverlight 2. Attached I am sending you just a definition of RadGridView using the latest SL2 binaries available where the group panel text is not truncated.
Additionally I would like to ask you for a sample reproducing your problem so I can be of more assistance.
On a side note - could you please send us the translated version of the grid so we can feature a Spanish translation in our localization example.
the Telerik team