From https://wordpress.org/support/topic/plugin-wpml-multilingual-cms-remember-browser-language-cookie-not-working

Go to plugins/sitepress-multilingual-cms/res/js/jquery.cookie.js

From line 34 you ca see that:

var days= options.expires, t = options.expires = new Date();
				t.setDate(t.getDate() + days);

Change to this :

var t = options.expires, t = options.expires = new Date();
var minutes = 60;
t.setTime(t.getTime() + (minutes * 60 * 1000));
Where : var minutes = 60 are the minutes you want until cookie expiration.