[formatter] add 'L' conversion

to map ISO 639-1 language codes to their full names

docs/formatting.md

@@ -116,6 +116,12 @@ Conversion specifiers allow to *convert* the value to a different form or type.
     <td><code>{tags!j}</code></td>
     <td><code>["sun", "tree", "water"]</code></td>
 </tr>
+<tr>
+    <td align="center"><code>L</code></td>
+    <td>Convert an <a href="https://en.wikipedia.org/wiki/ISO_639-1">ISO 639-1</a> language code to its full name</td>
+    <td><code>{lang!L}</code></td>
+    <td><code>English</code></td>
+</tr>
 <tr>
     <td align="center"><code>n</code></td>
     <td>Return the <a href="https://docs.python.org/3/library/functions.html#len" rel="nofollow">length</a> of a value</td>