TYPO3 redirects regarding base / leading to 404 when all languages are configured with language slug as base

How can I configure redirects within the TYPO3 v9 redirect module regarding the base / when all languages are configured with language slug as base /de/, /en/ and so on?

My Problem is a redirect with source_path like /something is resulting in a 404.

I checked with xdebug. The 404 happens because the processed $route does not have language information, which is set in config.yaml on key languages: but here not matching because of the defined base.

To make this more clear some examples:

• all existing pages are reachable like http://typo3.demo.local:3000/de/example1/ http://typo3.demo.local:3000/en/example1/ and so on
• plain base url http://typo3.demo.local:3000/ redirects to default language http://typo3.demo.local:3000/de/ by default behaviour
• within TYPO3 v9 redirect module a redirect should follow from http://typo3.demo.local:3000/test1/ to http://typo3.demo.local:3000/de/example1/ but there is no redirect, it shows the content from 404 page on http://typo3.demo.local:3000/test1/

This has been discussed for a while in https://forge.typo3.org/issues/87455#note-7

So switch to TYPO3 10 which has a feature switch or use a workaround like this I copied from the forge issue:

return [
    'frontend' => [
        'typo3/cms-redirects/redirecthandler' => [
            'disabled' => true,
        ],
        'hotfix-redirecthandler' => [
            'target' => \TYPO3\CMS\Redirects\Http\Middleware\RedirectHandler::class,
            'before' => [
                'typo3/cms-frontend/base-redirect-resolver',
            ],
        ],
    ], 
];

In an extension at Configuration/RequestMiddlewares.php