Add demo for locale plugin configuration

demos/locale/README.md

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+# Configure Locale Plugin
+
+## Sample Configuration
+
+Sample configuration for the [Locale plugin](https://plugins.jenkins.io/locale).
+
+This plugin allows you to set Jenkins UI language and ignore browser language preferences.
+
+### Minimal Configuration
+
+Set Jenkins UI to English:
+
+```yaml
+appearance:
+  locale:
+    systemLocale: "en"
+```
+
+### Full Configuration
+
+Set Jenkins UI to English and ignore browser Accept-Language header:
+
+```yaml
+appearance:
+  locale:
+    systemLocale: "en"
+    ignoreAcceptLanguage: true
+```
+
+## Options
+
+- `systemLocale`: The locale to use (e.g., "en", "en_US", "de", "fr", "ja", "zh_CN")
+- `ignoreAcceptLanguage`: If true, ignore browser's Accept-Language header and always use systemLocale
+
+## Notes
+
+- Without `ignoreAcceptLanguage: true`, Jenkins will still respect user's browser language settings
+- This is useful for teams that want consistent UI language regardless of browser settings
+- Supported locales depend on installed language packs
+

demos/locale/locale.yaml

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+---
+# Locale Plugin Configuration
+# Forces Jenkins UI to English and ignores browser language preferences
+
+appearance:
+  locale:
+    systemLocale: "en"
+    ignoreAcceptLanguage: true
+