diff --git a/docs/formatting.md b/docs/formatting.md
index d0e6c6a9..44a103d4 100644
--- a/docs/formatting.md
+++ b/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>
diff --git a/gallery_dl/formatter.py b/gallery_dl/formatter.py
index a84e7d34..d638fae9 100644
--- a/gallery_dl/formatter.py
+++ b/gallery_dl/formatter.py
@@ -485,6 +485,7 @@ _CONVERSIONS = {
     "j": util.json_dumps,
     "t": str.strip,
     "n": len,
+    "L": util.code_to_language,
     "T": util.datetime_to_timestamp_string,
     "d": text.parse_timestamp,
     "D": util.to_datetime,
diff --git a/test/test_formatter.py b/test/test_formatter.py
index 67632141..df9bb879 100644
--- a/test/test_formatter.py
+++ b/test/test_formatter.py
@@ -37,6 +37,7 @@ class TestFormatter(unittest.TestCase):
         "dt_dst": datetime.datetime(2010, 6, 1),
         "i_str": "12345",
         "f_str": "12.45",
+        "lang": "en",
         "name": "Name",
         "title1": "Title",
         "title2": "",
@@ -73,6 +74,9 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{l!j}", '["a","b","c"]')
         self._run_test("{dt!j}", '"2010-01-01 00:00:00"')
         self._run_test("{a!g}", "hello-world")
+        self._run_test("{lang!L}", "English")
+        self._run_test("{'fr'!L}", "French")
+        self._run_test("{a!L}", None)
         self._run_test("{a!n}", 11)
         self._run_test("{l!n}", 3)
         self._run_test("{d!n}", 3)
@@ -490,10 +494,10 @@ def noarg():
             fmt4 = formatter.parse("\fM " + path + ":lengths")
 
         self.assertEqual(fmt1.format_map(self.kwdict), "'Title' by Name")
-        self.assertEqual(fmt2.format_map(self.kwdict), "137")
+        self.assertEqual(fmt2.format_map(self.kwdict), "139")
 
         self.assertEqual(fmt3.format_map(self.kwdict), "'Title' by Name")
-        self.assertEqual(fmt4.format_map(self.kwdict), "137")
+        self.assertEqual(fmt4.format_map(self.kwdict), "139")
 
         with self.assertRaises(TypeError):
             self.assertEqual(fmt0.format_map(self.kwdict), "")