In an eZP 5.4 installation with a Symfony stacked frontend:

1. Edit ezpublish_legacy/settings/override/site.ini.append.php and add the following lines

   [URLTranslator]
   TransformationGroup=urlalias_iri
   

2. Clear the caches
3. Create a new article named "This has an Ä character"
4. Open it in the frontend
5. Confirm the design will fall back to legacy