Fix UI language switching bug (#21597) (#21749)

Backport #21597

Related:
* #21596 (comment)

There was a bug when switching language by AJAX: the irrelevant POST
requests were processed by the target page's handler.

Now, use GET instead of POST. The GET requests should be harmless.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/common-global.js

@@ -37,7 +37,7 @@ export function initHeadNavbarContentToggle() {
 export function initFootLanguageMenu() {
   function linkLanguageAction() {
     const $this = $(this);
-    $.post($this.data('url')).always(() => {
+    $.get($this.data('url')).always(() => {
       window.location.reload();
     });
   }