refactor: change meta class structure to include types.

Introduce requiredness of translations via implemented interface.

src/core/things/Message.ts

@@ -14,12 +14,13 @@
  */
 import {
   SCCreativeWork,
+  SCCreativeWorkMeta,
   SCCreativeWorkTranslatableProperties,
   SCCreativeWorkWithoutReferences,
 } from '../base/CreativeWork';
 import {SCThingThatCanBeOfferedTranslatableProperties} from '../base/ThingThatCanBeOffered';
 import {SCThingMeta, SCThingType} from '../Thing';
-import {SCTranslations} from '../types/i18n';
+import {SCMetaTranslations, SCTranslations} from '../types/i18n';
 import {SCISO8601Date} from '../types/Time';
 
 /**
@@ -69,12 +70,6 @@ export interface SCMessage extends SCCreativeWork, SCMessageWithoutReferences {
   type: SCThingType.Message;
 }
 
-/**
- * Meta information about a message
- */
-export class SCMessageMeta extends SCThingMeta {
-}
-
 /**
  * Audiences for messages
  */
@@ -93,3 +88,34 @@ export interface SCMessageTranslatableProperties
    */
   message?: string;
 }
+
+/**
+ * Meta information about messages
+ */
+export class SCMessageMeta extends SCThingMeta implements SCMetaTranslations<SCMessage> {
+  /**
+   * Translations of fields
+   */
+  fieldTranslations = {
+    de: {
+      ... SCCreativeWorkMeta.getInstance().fieldTranslations.de,
+    },
+    en: {
+      ... SCCreativeWorkMeta.getInstance().fieldTranslations.en,
+    },
+  };
+
+  /**
+   * Translations of values of fields
+   */
+  fieldValueTranslations  = {
+    de: {
+      ... SCCreativeWorkMeta.getInstance().fieldValueTranslations.de,
+      type: 'Nachricht',
+    },
+    en: {
+      ... SCCreativeWorkMeta.getInstance().fieldValueTranslations.en,
+      type: SCThingType.Message,
+    },
+  };
+}