Hi,
Ok for me it seems to work correctly. When I switch to English and navigate throughout the site, pages are in English and vice versa.
Your language is automatically detected but if you switch to another language, it is stored into your cookie until it expires or you change it manually.
Don't worry for SEO because we set the correct metas, check your source code:
<html lang="en">
<link rel="canonical" href="https://www.smartmydata.com/fichiers-ean.html?lng=en" />
<link rel="alternate" hreflang="fr" href="https://www.smartmydata.com/fichiers-ean.html?lng=fr">
<link rel="alternate" hreflang="x-default" href="https://www.smartmydata.com/fichiers-ean.html">
<link rel="alternate" hreflang="en" href="https://www.smartmydata.com/fichiers-ean.html?lng=en">