Use i18next for error message translantion

packages/knowledge-base-web-page-element-view/i18n/de/translation.json

 {
+  "error-head": "FEHLER: Information",
+  "is-forbidden": "ist verboten",
+  "troubled-server": "macht Probleme am Server",
+  "unknown-problems": "mit unbekanntem Problem",
+  "was-not-found": "wurde nicht gefunden"
 }

packages/knowledge-base-web-page-element-view/i18n/en/translation.json

 {
+  "error-head": "ERROR: information",
+  "is-forbidden": "is forbidden",
+  "troubled-server": "troubled server",
+  "unknown-problems": "with unknown problems",
+  "was-not-found": "was not found"
 }

packages/knowledge-base-web-page-element-view/vpu-kb-wpe-view.js

@@ -77,23 +77,20 @@ class VPUKnowledgeBaseWebPageElementView extends VPULitElement {
                 let status_msg;
                 switch (res.status) {
                     case 403:
-                        status_msg = that.lang === 'de' ? 'ist verboten' : 'is forbidden';
+                        status_msg = i18n.t('is-forbidden');
                         break;
                     case 404:
-                        status_msg = that.lang === 'de' ? 'wurde nicht gefunden' : 'was not found';
+                        status_msg = i18n.t('was-not-found');
                         break;
                     case 500:
-                        status_msg = that.lang === 'de' ? 'macht Probleme am Server' : 'troubled server';
+                        status_msg = i18n.t('troubled-server');
                         break;
                     default:
-                        status_msg = that.lang === 'de' ? 'mit unbekanntem Problem' : 'with unknown problems';
+                        status_msg = i18n.t('unknown-problems');
                 }
-                if (that.lang === 'de') {
-                    that.error = html`<p>FEHLER: Information "<b>${that.value}</b>" ${status_msg} (${res.status}).</p>`;
-                } else {
-                    that.error = html`<p>ERROR: information "<b>${that.value}</b>" ${status_msg} (${res.status}).</p>`;
-                }
-                throw new Error('HTTP error: ' +that.value + ' ' + status_msg + ', status = ' + res.status);
+                const error_head = i18n.t('error-head');
+                that.error = html`<p>${error_head} "<b>${that.value}</b>" ${status_msg} (${res.status}).</p>`;
+                throw new Error('HTTP ' + error_head + ' ' + that.value + ' ' + status_msg + ', status = ' + res.status);
             }
             return res.json();
         })