Revert lost translations in es, gl, ja

Looks like Jacques needs to enable these locales on his system.

docs/ja_JP.UTF-8/html/EXTENDING.html

@@ -18,15 +18,15 @@
             width="205"
             height="210"
             alt="Tux Paint"><br>
-        version 0.9.27      </h1>
+        バージョン 0.9.27      </h1>
 
       <p>
-        Copyright &copy; 2002-2021 by various contributors; see AUTHORS.<br>
+        Copyright &copy; 2002-2021 by various contributors; "AUTHORS" 参照.<br>
         <a href="http://www.tuxpaint.org/">http://www.tuxpaint.org/</a>
       </p>
 
       <p>
-        September  6, 2021      </p>
+        2021年9月 6日      </p>
     </center>
 
     <hr size="2"
@@ -35,10 +35,10 @@
     <table border="2"
            cellspacing="0"
            cellpadding="2"
-           summary="Table of Contents">
+           summary="目次">
       <tr>
         <th>
-          Table of Contents        </th>
+          目次        </th>
       </tr>
 
       <tr>
@@ -474,7 +474,7 @@
             If no translation is available for the language Tux Paint is currently running in, the US English text is used.          </p>
         </blockquote>
 
-        <h4>Windows Users</h4>
+        <h4>Windows のユーザー</h4>
 
         <blockquote>
           <p>
@@ -673,7 +673,7 @@
           </ul>
         </blockquote>
 
-        <h4>Windows Users</h4>
+        <h4>Windows のユーザー</h4>
 
         <blockquote>
           <p>