I'm assuming /fr/ is for language and /park/ is the slug of the Locations entry? Is /brussels/ also the slug for another entry, or is it just a key you're using to determine what content to show on the Park page?
In config/routes.php you can set up a route for the French site like this:
return [
'fr' => [
<citySlug>/<entrySlug:{slug}>' => ['template' => '_location']
]
];
And then in the _location.twig template, you'll be able to access citySlug variable (e.g. 'new-york'). You could then use that to find and display the relevant content. How you do that will depend on whether that content lives on the Park entry, or if you have to query some other entry for it.
Just note that because there's no static base URL segment here (e.g. /fr/locations/new-york/park/) you might have to do some additional logic to make sure the route doesn't unintentionally affect other sections.
EDIT: I may have misunderstood the intent here. Is this just entries in the locations section that have slightly different content based on the city? Or is it more like each city has totally different content for the same (or a similar) set of entries? You may actually just want to create multiple sites (one for each language/city combo) instead of doing the above.