The country names on the terrain map were part of the map imagery (before they disappeared). Unfortunately there seems to be no way to request different map imagery that includes country names.
The only option I see is to define your own map overlay on top of the terrain map, with all the country names. (Probably not practical and not easy).
1st EDIT: As suggested in a comment below, if you plan to add large overlays over the terrain map, you may want to use the open-source tool MapTiler (for Windows and Mac OS X). MapTiler will automate the tiling process for you, and in addition, once the tiling process is finished, it will generate a simple HTML viewer. If you are adding the name of just one country, you may want to use the GGroundOverlay instead. (Google Demo)
2nd EDIT: Note that this issue has been acknowledged by a Google employee on January 20th, and therefore you may want to consider waiting for a few days to see if it gets sorted automatically, before you commit your time to fix it.
3nd EDIT: This has been fixed a few weeks ago by Google.