I like this feature a lot, but I do not know how it is implemented and whether it would be possible to have users change the range. Maybe it is hardcoded for some performance reason, I have honestly no clue. But I am giving below a few paths that could help one eventually improve the system.
The page https://en.wikipedia.org/wiki/Special:Nearby is powered by the MediaWiki extension https://www.mediawiki.org/wiki/Extension:NearbyPages .
The source code is https://gerrit.wikimedia.org/g/mediawiki/extensions/NearbyPa... (GitHub mirror https://github.com/wikimedia/mediawiki-extensions-NearbyPage... ).
Issues/bugs are tracked at https://phabricator.wikimedia.org/tag/nearbypages/ , it is possible a similar request already exists there.
Do note I am not in the development / product side and I have no view about their backlog or whether they have the bandwidth to review such an incoming review request or a patch.
That extension exposes a public API that allows setting any search radius up to 10 km ( https://en.wikipedia.org/w/api.php?action=help&modules=query... ), so presumably there's no barrier to making the search radius smaller, but maybe there'd be performance issues if you allowed making it larger.
(NearbyPages just embeds a Vue app into the page that makes a client-side fetch() call to invoke that API with a hard-coded radius.)
Now I'm not sure if switching the language pulls in more results or just changes the article language...