feat(all)!: Multi language support (#1081)

## What does this do?

Implements multi-language support based on this [design
doc](https://www.notion.so/Multilingual-Support-Design-3481f8f2e65e8076ad40f613813c66dc?source=copy_link).

## How was it tested?

Added tests and ran integration tests locally against this branch.

## Here's a fun image for your troubles


<img width="539" height="960" alt="image"
src="https://github.com/user-attachments/assets/45b7da94-912a-472d-b6e5-1fc54aaabc0e"
/>

Author: bagelbits

.gitignore

@@ -4,6 +4,7 @@ named-*
 /node_modules/
 
 /.idea
+/.claude
 
 **/built/
 **/built/**

CONTRIBUTING.md

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+# Contributing to 5e-database
+
+Thanks for helping improve the 5e SRD database! This guide covers the two main ways to contribute: **data corrections** and **translations**.
+
+---
+
+## General workflow
+
+1. Fork this repository
+2. Create a branch for your changes
+3. Open a pull request — incomplete work is welcome, we can help get it ready
+
+We use [Semantic Release](https://semantic-release.gitbook.io/semantic-release/), so please follow conventional commit message conventions for your PR title:
+
+| PR title prefix                       | Effect        |
+| ------------------------------------- | ------------- |
+| `fix(...)`                            | Patch release |
+| `feat(...)`                           | Minor release |
+| `feat(...) + BREAKING CHANGE:` footer | Major release |
+
+---
+
+## Correcting English data
+
+English source files live in `src/{year}/en/`. Edit the appropriate `5e-SRD-{Collection}.json` file and open a PR. CI will validate your changes automatically.
+
+---
+
+## Contributing translations
+
+Translations are entirely community-driven. Adding a translation requires no database access or special tooling — only a text editor and a fork.
+
+### File location
+
+Create a file at:
+
+```
+src/{year}/{lang}/5e-SRD-{Collection}.json
+```
+
+where `{lang}` is a [BCP 47](https://www.rfc-editor.org/rfc/rfc5646) language tag (e.g. `de`, `fr`, `pt-BR`, `es-419`, `zh-Hans`).
+
+**Examples:**
+```
+src/2014/de/5e-SRD-Spells.json
+src/2024/fr/5e-SRD-Equipment.json
+src/2024/pt-BR/5e-SRD-Conditions.json
+```
+
+### File format
+
+A translation file is a JSON array. Each entry must include `index` (to identify the source record) plus **only the fields you are translating**. You do not need to include every entry or every field — partial translations are fully supported.
+
+```json
+[
+  {
+    "index": "acid-arrow",
+    "name": "Säurepfeil",
+    "desc": [
+      "Ein glitzernder grüner Pfeil schießt auf ein Ziel..."
+    ],
+    "higher_level": [
+      "Wenn du diesen Zauber mit einem Zauberplatz der 3. Stufe oder höher wirkst..."
+    ]
+  }
+]
+```
+
+### Translatable fields
+
+Only text fields may be translated. Structural fields (`url`, numeric values, and API references) must be omitted. `index` is required for matching but must not be translated — its value must be identical to the English source. The exact set of translatable fields depends on the collection — any text field present in the English entry for that `index` is allowed.
+
+The table below shows common examples from the Spells collection:
+
+| Field                 | Translatable                                                |
+| --------------------- | ----------------------------------------------------------- |
+| `name`                | Yes                                                         |
+| `desc`                | Yes                                                         |
+| `higher_level`        | Yes                                                         |
+| `index`               | No — include it for matching, but don't translate its value |
+| `url`                 | No                                                          |
+| Numeric values        | No                                                          |
+| API reference objects | No                                                          |
+
+Other collections expose different text fields (e.g. `summary` on subclasses, `description` on some entries). Include any text field that appears in the English source — CI will reject fields that don't exist there.
+
+### Validation rules
+
+The test suite (`npm test`, also run by CI on every PR) will fail if your translation file:
+
+- Contains an `index` that does not exist in the corresponding English file
+- Contains a field that does not exist in the English entry for that `index`
+- Contains duplicate `index` values
+- Produces an invalid document when translation fields are merged with the English entry (Zod schema validation)
+
+Run `npm test` locally before opening a PR to catch these issues early.
+
+### Partial translations are fine
+
+You do not need to translate every entry or every field. The API will fall back to English on a per-field basis for anything not covered by a translation. A file covering even a handful of entries is a useful contribution.
+
+---
+
+## Code of Conduct
+
+See the [Code of Conduct](https://github.com/5e-bits/5e-database/wiki/Code-of-Conduct).