Issue #3193793: Add option to set cookie client side

assets/js/language_cookie.set-cookie.js

+/**
+ * @file
+ * Behavior for setting the language_cookie cookie client-side.
+ */
+
+(function (drupalSettings, Cookies) {
+  "use strict";
+
+  if (typeof drupalSettings.language_cookie !== 'undefined') {
+    let config = drupalSettings.language_cookie;
+    let currentCookie = Cookies.get(config.cookie_name);
+    if (config.set_on_every_pageload || !currentCookie || !currentCookie.length || currentCookie !== config.cookie_value) {
+      Cookies.set(config.cookie_name, config.cookie_value, config.options);
+    }
+  }
+
+})
+(drupalSettings, window.Cookies);