Fix language filter

src/main/webapp/app/shared/model/search/search-input.model.ts

-import { MetadataSearchInput } from 'app/shared/model/search/metadata-search-input.model';
 import { ILanguages } from 'app/shared/model/exercise.model';
+import { MetadataSearchInput } from 'app/shared/model/search/metadata-search-input.model';
 
 export class SearchInput {
   public fulltextQuery: string;
@@ -23,9 +23,12 @@ export class SearchInput {
     this.metadata.parentId = value.parentId;
 
     this.metadata.naturalLanguage = [];
-    for(const eType of Object.values(ILanguages)) {
+    Object.keys(ILanguages).forEach(typeName => {
+      if (value[typeName]) {
+        const eType = ILanguages[typeName as keyof typeof ILanguages];
         this.metadata.naturalLanguage.push(eType);
-    }
+      }
+    });
 
     this.metadata.learningResourceType = '';
   }