In TA, you need to go to your My Documents\My Games\GC2TwilightArnor folder and look for the race config files.  They'll have an extension of .raceconfigxml.  For example, the Altarian race configuration file is called Altarian Resistance.raceconfigxml.

Delete these files and you'll be back to default.

Some might have an extension of .customracexml as well if they follow DA syntax, but I don't think they do.  Nevertheless, if you see one with a .customracexml extension, delete it as well.