The 21 Merchant Patterns, and a Language Button You Can Read
Merchant stock in Nightreign is not rolled fresh and it has nothing to do with the map seed. It comes from 21 fixed patterns, and a run uses one of them. Merchant Patterns is all 21, written out.
The part that makes it worth having is that the pattern is shared. All four merchants in a run are on the same one, so the first Normal merchant you walk past tells you what the Township merchant is holding on the other side of the map, and what the Legendary merchant will have when it opens on day 2. That is the question the page is for: whether the trip is worth making.
How the page reads

The 21 patterns run down the left, each one shown by three item icons taken from the end of its Normal, Township and Legendary stock. You find yours by matching what is on the counter in front of you against those icons, then read the rest of the pattern on the right.
A toggle at the top swaps the whole catalogue between a casual expedition and the Deep of Night, since the stock is not the same in both. The search box runs over item names, skills and effects: type one in and the patterns carrying it light up in the list, and so do the matching items inside them.
Hovering an item raises a card with its skill and its effects, magnitudes included.

Frozen Needle there carries Hoarfrost Stomp and three lines. The arrow follows the sign of the number, and the colour follows what the line does to you. Improved Charge Attacks at +22% is green and pointing up. Multiple Periodical Glintblades has no number, so it gets neither. Ailments Cause Increased Damage at +25% points up as well and is red, because that line is a curse and the plus is against you.
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Which merchant is where
Normal merchants stand at every fort, every main encampment and every castle, and every expedition has them. A Township merchant is one per expedition at most, and plenty of seeds have none at all. The Legendary merchant exists only in the Great Hollow, only from day 2, and charges the same price for everything it sells. The fourth waits at the Nightlord door, and the only part of its stock the pattern moves is its three Crystal Tears.
One limit worth saying out loud: the catalogue covers the stock that changes from pattern to pattern, and nothing else. The items a merchant always carries are the same in every run, so listing them 21 times would tell you nothing about which pattern you are in.
The catalogue started from merchant data shared by Marco1522_ and ababab777. What is published has since been condensed, corrected and completed here. If something on it does not match what you see on the counter, tell me and I will fix it.
The names come from the game
The page's own writing is in all ten languages. The item names are not translated by me either: weapons, skills and spells come out of Elden Ring's text, which is where Nightreign takes them from.
Arabic is the exception, and it is not one I can fix. Elden Ring never shipped an Arabic localisation, so there is no Arabic name to pull for most of this. In Arabic, 143 of the 576 names resolve and the rest stay in English. The other eight languages resolve all 576.
Deep of Night stays in English everywhere for a different reason: no game text carries it, and the name of a game mode is not a thing to guess at.
The front page has a third button now, Merchants, between Events and How to Use. Special Events lost its adjective on the way and reads Events.
The word Language is never translated now
Someone wrote in to say his page was in Japanese and he could not get out of it, because the way out was written in Japanese too.
The buttons read Language (ja), Language (pt-br), Language (en), and so on. The word stays in English wherever it appears, so the way out is always in an alphabet you can read. The list itself is unchanged and still opens in each language's own name: 日本語, العربية, Deutsch, 한국어. On a seed result page the Settings button carries that same label rather than having a button of its own.
The screen stays awake on a phone
This went in quietly during 1.6.08 and I never announced it, so I am announcing it now. It is a thing that should have been in the project from the start. It took this long for one reason only, which is that I did not think of it sooner.
On a phone, the screen no longer dims while a seed result map is open. The switch is in Map Settings, it is on by default, and it appears on phones only. It lets go after 40 minutes with no touches, so a map left open on the table does not empty your battery.