I have this setup for culture and hostnames:

"https://www.bmsilo.com/fr/" should be removed, but for now its needed since when I go to www.bmsilo.fr it renders English variant of content. Or as another example; when I go to https://www.bmsilo.fr/produits/silos I get 404 error (again; it tries to resolve to Enlgish content, and that is on url https://www.bmsilo.com/products/silos), but for https://www.bmsilo.com/fr/produits/silos I correctly get French content.

Why is that?

In my Program.cs I have this setup that might be relevant:

if (app.Environment.IsProduction())
{
    app.Use(async (context, next) =>
    {
        var request = context.Request;
        var host = request.Host.Value.ToLower();
        var path = request.Path.Value ?? string.Empty;
        var scheme = request.Scheme;

        // Ensure SSL
        if (scheme == "http" && !context.Request.Headers["X-Forwarded-Proto"].ToString().Equals("https", StringComparison.OrdinalIgnoreCase))
        {
            var newUrl = $"https://{host}{path}{request.QueryString}";
            context.Response.Redirect(newUrl, permanent: true);
            return;
        }

        // Ensure www
        if (!host.StartsWith("www."))
        {
            var newUrl = $"{scheme}://www.{host}{path}{request.QueryString}";
            context.Response.Redirect(newUrl, permanent: true);
            return;
        }

        await next();
    });
}

...