Add locale plugin demo README

.vscode/settings.json

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+{
+    "java.compile.nullAnalysis.mode": "automatic"
+}

demos/locale/README.md

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+# Configure Locale plugin
+
+Basic configuration of the [Locale plugin](https://plugins.jenkins.io/locale)
+
+The Locale plugin allows administrators to control the language settings of the Jenkins user interface, overriding browser preferences and setting a default language for all users.
+
+## sample configuration
+
+```yaml
+appearance:
+  locale:
+    systemLocale: en
+    ignoreAcceptLanguage: true
+    allowUserPreferences: false
+```
+
+### Configuration options
+
+- `systemLocale`: The default language code to use for the Jenkins UI (e.g., `en`, `fr`, `de`, `ja`, `zh_CN`, `de_AT`)
+- `ignoreAcceptLanguage`: If `true`, ignores browser language preferences and forces all users to use the system locale
+- `allowUserPreferences`: If `true`, allows users to set their own language preferences. If `false`, users must use the system locale
+
+### Example: Force English for all users
+
+This configuration forces all users to use English, ignoring browser preferences and not allowing user preferences:
+
+```yaml
+appearance:
+  locale:
+    systemLocale: en
+    ignoreAcceptLanguage: true
+    allowUserPreferences: false
+```
+
+### Example: Default to French but allow user preferences
+
+This configuration sets French as the default but allows users to override it:
+
+```yaml
+appearance:
+  locale:
+    systemLocale: fr
+    ignoreAcceptLanguage: false
+    allowUserPreferences: true
+```
+
+### Example: Default to German (Austria) with browser preference fallback
+
+This configuration sets Austrian German as default but respects browser preferences if not forced:
+
+```yaml
+appearance:
+  locale:
+    systemLocale: de_AT
+    ignoreAcceptLanguage: false
+    allowUserPreferences: true
+```
+