Step 1. Tools > Import and Export Settings...
Step 2. "Import Selected Environment Settings"
Step 3. "No, import settings, overwrite.."
Step 4. Pick "Visual C# Development Settings", or "Visual Basic Development Settings", or whichever Default Settings you want to change the default language to.
Step 5. *Clear* the checkbox next to "All Settings". Now you're importing nothing at all, and overwriting nothing at all.
Step 6. Check "All Settings > General Settings > New Project Dialog Preferred Language". Now your importing just that one setting, and leaving everything else as-is.