How to get a page slug in WordPress?

Going by your link tag and the place you want the slug in, the code would be like this.

<link rel="alternate" hreflang="fr-BE" href="https://www.moneytrans.eu/belgium/<?= get_post_field('post_name'); ?>" />

You don’t need to add all those spaces in attributes, for example, instead of rel = "alternate" do rel="alternate", and so on.