Made the language selector button visible

After upgrading my OS to Ubuntu 22.04 the language selector button
didn't show up in the viewer taskbar. Investigation shows that the id
used in the CSS was applied to the wrong HTML element (the enclosing
<a> rather than <img>).

static/viewer.html

@@ -34,12 +34,12 @@
     <div class="kiwix" style="display:none" id="kiwixtoolbarwrapper">
       <div id="kiwixtoolbar" class="ui-widget-header">
         <div class="kiwix_centered">
-          <a id="uiLanguageSelectorButton"
-             onclick="window.modalUILanguageSelector.show()"
+          <a onclick="window.modalUILanguageSelector.show()"
              alt="Select UI language"
              aria-label="Select UI language"
              title="Select UI language">
-            <img src="./skin/langSelector.svg?KIWIXCACHEID">
+            <img id="uiLanguageSelectorButton"
+                 src="./skin/langSelector.svg?KIWIXCACHEID">
           </a>
           <div class="kiwix_searchform">
             <form class="kiwixsearch" method="GET" action="javascript:performSearch()" id="kiwixsearchform">