docs(localization): add translation guide for contributors

Add a TRANSLATION_GUIDE.md to the localization package that documents:
- Current supported languages
- Step-by-step instructions for adding a new language
- How to register translations in locales.ts and messages.ts
- Tips for translators (preserving placeholders, finding missing keys)
- Translation structure and how keys are referenced in components

This lowers the barrier for community members to contribute translations.

Fixes #3000

Signed-off-by: pierreeurope <pierre.europe@pm.me>

Author: pierreeurope

src/localization/TRANSLATION_GUIDE.md

@@ -0,0 +1,121 @@
+# Translation Guide for kepler.gl
+
+Thank you for helping translate kepler.gl! This guide explains how to add a new language or update existing translations.
+
+## Current Languages
+
+| Code | Language | File |
+|------|----------|------|
+| `en` | English | `src/translations/en.ts` |
+| `fi` | Suomi (Finnish) | `src/translations/fi.ts` |
+| `pt` | Português (Portuguese) | `src/translations/pt.ts` |
+| `es` | Español (Spanish) | `src/translations/es.ts` |
+| `ca` | Català (Catalan) | `src/translations/ca.ts` |
+| `ja` | 日本語 (Japanese) | `src/translations/ja.ts` |
+| `cn` | 简体中文 (Simplified Chinese) | `src/translations/cn.ts` |
+| `ru` | Русский (Russian) | `src/translations/ru.ts` |
+
+## Adding a New Language
+
+### 1. Create the translation file
+
+Copy `src/translations/en.ts` to `src/translations/<code>.ts` (use the [ISO 639-1](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) two-letter code):
+
+```bash
+cp src/translations/en.ts src/translations/de.ts
+```
+
+### 2. Translate all strings
+
+Edit your new file and translate every string value. Keep the keys unchanged:
+
+```typescript
+// src/translations/de.ts
+import {LOCALES} from '../locales';
+
+export default {
+  property: {
+    weight: 'Gewicht',
+    label: 'Beschriftung',
+    fillColor: 'Füllfarbe',
+    // ... translate all entries
+  },
+  // ...
+};
+```
+
+**Tips:**
+- Use `src/translations/en.ts` as the reference for all keys
+- Keep placeholders like `{mapboxToken}` and `{errorMessage}` unchanged
+- Preserve HTML entities and formatting strings
+- For keys you're unsure about, leave the English text and add a `// TODO: translate` comment
+
+### 3. Register the language
+
+**a.** Add the locale to `src/locales.ts`:
+
+```typescript
+export const LOCALES = {
+  en: 'English',
+  // ... existing locales
+  de: 'Deutsch'  // <-- add your language
+};
+```
+
+**b.** Import and add the translation in `src/messages.ts`:
+
+```typescript
+import de from './translations/de';  // <-- add import
+
+export const messages = {
+  // ... existing entries
+  de: flattenMessages(de)  // <-- add entry
+};
+```
+
+### 4. Test your translation
+
+Set the locale in a kepler.gl instance:
+
+```javascript
+import {LOCALE_CODES} from '@kepler.gl/localization';
+
+const customizedKeplerGlReducer = keplerGlReducer
+  .initialState({
+    uiState: {
+      locale: LOCALE_CODES.de
+    }
+  });
+```
+
+Or switch languages at runtime using the locale panel in the map controls (globe icon).
+
+## Updating Existing Translations
+
+When new English strings are added, other translation files may fall behind. To find missing translations:
+
+1. Compare your translation file against `en.ts` to identify missing keys
+2. Search for `// TODO` comments in existing translation files
+3. Check for keys that still have English values in non-English files
+
+## Translation Structure
+
+Translations use a nested object structure that gets flattened by `flattenMessages()`:
+
+```typescript
+{
+  property: {
+    weight: 'weight',     // becomes 'property.weight'
+    label: 'label'        // becomes 'property.label'
+  },
+  toolbar: {
+    exportImage: 'Export Image'  // becomes 'toolbar.exportImage'
+  }
+}
+```
+
+In components, strings are referenced via `<FormattedMessage id="property.weight" />` or the `intl.formatMessage()` API.
+
+## Questions?
+
+Open an issue on [GitHub](https://github.com/keplergl/kepler.gl/issues) with the `localization` label.