You'd have to look at the controls source code within that directory. You'll find code like so
rfvEmailRequired.ErrorMessage = GetString("board.subscription.noemail");
I should mention there are .resx files within the system which is where these default resource strings are created and stored BUT when you use the Localization app and add the same key, the key value you store in the Localization app takes priority over the value in the .resx file. So always best to use the Localization app to override the defaults so you're not changing the base code.