Hmm I see wow. Some text errors are handled softly, by "unlocalized text", and others just crap out and you don't know what caused it. Alright I'll edit religions.txt, no biggie. Meantime, we're trying to figure this out also -- if you add a new culture properly, it works and everything's fine, but then if you double-click on it as another culture, it craps out. What do you think?
My thinking is that the text strings that show up when clicking another settlement (e.g. "this settlement can hold out for 10 turns") don't exist for the new culture, hence the crap out. It would be strange for such text strings to be specified by culture but hey, worth a try to fix the ctd. But, get this, I just did a search through all 10 gigs of this game, there is no such string "can hold out" anywhere!
EDIT Wow, Alpaca be thanked, settlement details now show up with my custom religion. The fix for this is so trivial, but completely crashes without it -- need to add "{newreligion} Newreligion" to /text/religions.txt.
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