Add mod documentation in seven languages

docs/en.md

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+# Quests Engine — quests, their steps and the journal
+
+Quests Engine keeps track of the player's tasks. A quest has a name and a numbered list of steps; scenarios start it, move it to the next step, mark things with tags and finish it. The **Journal** panel shows the quests the player has taken, and the game saves their state.
+
+It is used by Cthangover (the prologue's awakening and the old house) and by RPG Plus (the woodland quests).
+
+## The journal
+
+Open it with the journal icon on the toolbar. Before the player has taken any quest, it is empty:
+
+![The Journal in RPG Plus at the start: Active and Completed tabs, "No quests" on both pages.](assets/journal-before.jpg)
+
+In RPG Plus the elder of the village gives the first quest. Ask him where to look:
+
+![The elder's house: the hero in front of the elder, two answers on the right — "Can a wolf put out lamps?" and "Where do I find it?".](assets/elder-choice.jpg)
+
+After the answer the quest is in the journal:
+
+![The Journal after the talk: "A light on the ledger" in the Active list; on the right "Entry II of V", the first step struck through and the current step "Defeat the wolf by the cave in the eastern forest. Find the amber heart."](assets/journal-after.jpg)
+
+- **Active** lists quests in progress, **Completed** the finished ones. Quests not yet started never appear.
+- The right page shows the selected quest: steps already done are struck through, the current step is marked, and later steps stay hidden, so the journal never gives the story away.
+- Tags the quest has earned are shown as small gold labels under the entry.
+
+## A quest definition
+
+Quests are JSON files of kind `quest` in a mod's `quests/` folder. This is the quest from the pictures (from RPG Plus):
+
+```json
+{
+  "Kind": "quest",
+  "Items": [
+    {
+      "Id": "woodland_ward",
+      "Name": "rpg_plus/quest/ward/name",
+      "StatusToDescription": {
+        "0": "rpg_plus/quest/ward/0",
+        "1": "rpg_plus/quest/ward/1",
+        "2": "rpg_plus/quest/ward/2",
+        "3": "rpg_plus/quest/ward/3",
+        "4": "rpg_plus/quest/ward/4"
+      }
+    }
+  ]
+}
+```
+
+`Name` and every step are translation keys. The numbers are the quest's **step**: step 1 shows the text of `1` as current and `0` struck through — "Entry II of V" in the picture.
+
+## Moving a quest from a scenario
+
+The elder's answer in RPG Plus starts the quest and puts it on step 1:
+
+```scenario
+action quest.set_status quest_id=woodland_ward status=Progress
+action quest.set_data_status quest_id=woodland_ward level=1
+```
+
+- `quest.set_status` — the quest's state: `NotStarted`, `Progress` (active) or `End` (completed).
+- `quest.set_data_status` — the current step, `level` is its number.
+- `quest.add_tag` / `quest.remove_tag` with `tag=…` — remember something that happened, for example which item the player took first.
+- `quest.send_notification` — shows the player a notification about the quest; changing a quest does not do this by itself.
+
+## Checking a quest
+
+Quest checks work wherever the game takes a condition: `when`, answer conditions, scene conditions and map events.
+
+```scenario
+when "woodland_ward.DataStatus >= 2" -> :lantern
+```
+
+- `<quest>.IsActive` — the quest is in progress;
+- `<quest>.Status` — `0` not started, `1` in progress, `2` completed;
+- `<quest>.DataStatus` — the current step;
+- `<quest>.hasTag("tag")`, `<quest>.notHasTag("tag")` — Cthangover uses these to pick which scenario of a scene plays, for example `AwakeningQuest.hasTag("book_taken")`.
+
+## If something goes wrong
+
+1. **The quest is not in the journal.** Its state must be `Progress` (or `End` for the Completed tab); setting only the step is not enough.
+2. **The journal shows a key instead of text.** Add the step's key to the mod's `locale` files.
+3. **A condition is always false.** The quest id is spelled differently from the `Id` in the quest file; the log warns about an unknown quest.