diff --git a/wagtail_localize/machine_translators/deepl.py b/wagtail_localize/machine_translators/deepl.py
index 3de38fade..1d6ce5766 100644
--- a/wagtail_localize/machine_translators/deepl.py
+++ b/wagtail_localize/machine_translators/deepl.py
@@ -6,11 +6,13 @@
 
 
 def language_code(code, is_target=False):
-    # DeepL supports targeting Brazillian Portuguese but doesn't have this for other languages
-    if is_target and code in ["pt-pt", "pt-br"]:
-        return code
+    # DeepL supports targeting Brazilian Portuguese and requires to specifically request American or British English.
+    # @see https://www.deepl.com/en/docs-api/translate-text/translate-text
+    upper_code = code.upper()
+    if is_target and upper_code in ["PT-PT", "PT-BR", "EN-US", "EN-GB"]:
+        return upper_code
 
-    return code.split("-")[0].upper()
+    return upper_code.split("-")[0]
 
 
 class DeepLTranslator(BaseMachineTranslator):
diff --git a/wagtail_localize/machine_translators/tests/test_deepl_translator.py b/wagtail_localize/machine_translators/tests/test_deepl_translator.py
index c417848b2..5cfa8b4b7 100644
--- a/wagtail_localize/machine_translators/tests/test_deepl_translator.py
+++ b/wagtail_localize/machine_translators/tests/test_deepl_translator.py
@@ -1,7 +1,7 @@
 from django.test import TestCase, override_settings
 
 from wagtail_localize.machine_translators import get_machine_translator
-from wagtail_localize.machine_translators.deepl import DeepLTranslator
+from wagtail_localize.machine_translators.deepl import DeepLTranslator, language_code
 
 
 DEEPL_SETTINGS_FREE_ENDPOINT = {
@@ -29,3 +29,27 @@ def test_paid_api_endpoint(self):
         self.assertIsInstance(translator, DeepLTranslator)
         paid_api_endpoint = translator.get_api_endpoint()
         self.assertEqual(paid_api_endpoint, "https://api.deepl.com/v2/translate")
+
+    def test_language_code_as_source(self):
+        mapping = {
+            "pt-pt": "PT",
+            "pt-br": "PT",
+            "en-us": "EN",
+            "en-gb": "EN",
+            "es-es": "ES",
+        }
+        for code, expected_value in mapping.items():
+            with self.subTest(f"Testing language_code with {code}"):
+                self.assertEqual(expected_value, language_code(code, is_target=False))
+
+    def test_language_code_as_target(self):
+        mapping = {
+            "pt-pt": "PT-PT",
+            "pt-br": "PT-BR",
+            "en-us": "EN-US",
+            "en-gb": "EN-GB",
+            "es-es": "ES",
+        }
+        for code, expected_value in mapping.items():
+            with self.subTest(f"Testing language_code with {code} as target"):
+                self.assertEqual(expected_value, language_code(code, is_target=True))