Add Slovenian language for frontend

See #303

src-ui/src/app/services/settings.service.ts

@@ -252,6 +252,12 @@ export class SettingsService {
         englishName: 'Russian',
         dateInputFormat: 'dd.mm.yyyy',
       },
+      {
+        code: 'sl-si',
+        name: $localize`Slovenian`,
+        englishName: 'Slovenian',
+        dateInputFormat: 'dd.mm.yyyy',
+      },
       {
         code: 'sv-se',
         name: $localize`Swedish`,