This is dpne as a two-stage approach. TXT_LOGTEXTxxx will always take precedence over the log lumps, and TXT_ILOGxxx will only replace the original IWAD content.
This is so that PWADs replacing these lumps don't get overridden by the default texts.
Now all this content can be localized. However, since this is actual game content it was placed in a secondary file in zd_extra.pk3, so that it won't affect the GPL-compatible status of the main one.