diff --git a/libretranslate/app.py b/libretranslate/app.py
index d2a9415..69752b6 100644
--- a/libretranslate/app.py
+++ b/libretranslate/app.py
@@ -789,17 +789,18 @@ def create_app(args):
           if source_lang == "auto":
               candidate_langs = detect_languages(src_texts)
               detected_src_lang = candidate_langs[0]
+              src_lang = get_language_with_fallback(detected_src_lang["language"], languages)
           else:
               detected_src_lang = {"confidence": 100.0, "language": source_lang}
+              src_lang = next((l for l in languages if l.code == source_lang), None)
         else:
           detected_src_lang = {"confidence": 0.0, "language": "en"}
-
-        src_lang = get_language_with_fallback(detected_src_lang["language"], languages)
+          src_lang = next((l for l in languages if l.code == "en"), None)
 
         if src_lang is None:
             abort(400, description=_("%(lang)s is not supported", lang=source_lang))
 
-        tgt_lang = get_language_with_fallback(target_lang, languages)
+        tgt_lang = next((l for l in languages if l.code == target_lang), None)
 
         if tgt_lang is None:
             abort(400, description=_("%(lang)s is not supported",lang=target_lang))
@@ -977,12 +978,12 @@ def create_app(args):
         if os.path.splitext(file.filename)[1] not in frontend_argos_supported_files_format:
             abort(400, description=_("Invalid request: file format not supported"))
 
-        src_lang = get_language_with_fallback(source_lang, languages)
+        src_lang = next((l for l in languages if l.code == source_lang), None)
 
         if src_lang is None and source_lang != "auto":
             abort(400, description=_("%(lang)s is not supported", lang=source_lang))
 
-        tgt_lang = get_language_with_fallback(target_lang, languages)
+        tgt_lang = next((l for l in languages if l.code == target_lang), None)
 
         if tgt_lang is None:
             abort(400, description=_("%(lang)s is not supported", lang=target_lang))
diff --git a/libretranslate/language.py b/libretranslate/language.py
index cc7ae24..a2f3d91 100644
--- a/libretranslate/language.py
+++ b/libretranslate/language.py
@@ -54,7 +54,9 @@ def get_language_with_fallback(lang_code, languages):
     
     language_variants = {
         'pt': ['pb'],
-        'pb': ['pt']
+        'pb': ['pt'],
+        'zh': ['zt'],
+        'zt': ['zh']
     }
     
     fallbacks = language_variants.get(lang_code, [])
diff --git a/libretranslate/tests/test_api/test_language_variant_fallback.py b/libretranslate/tests/test_api/test_language_variant_fallback.py
index 5a9a58d..e9ab858 100644
--- a/libretranslate/tests/test_api/test_language_variant_fallback.py
+++ b/libretranslate/tests/test_api/test_language_variant_fallback.py
@@ -29,11 +29,11 @@ def client_with_pt(app_with_pt):
     return app_with_pt.test_client()
 
 
-def test_portuguese_fallback_pb_to_pt(client_with_pt):
-    response = client_with_pt.post("/translate", data={
-        "q": "Hello",
-        "source": "en",
-        "target": "pb",
+def test_auto_detect_fallback_pt_to_pb(client_with_pb):
+    response = client_with_pb.post("/translate", data={
+        "q": "Olá mundo",
+        "source": "auto",
+        "target": "en",
         "format": "text"
     })
     
@@ -43,7 +43,21 @@ def test_portuguese_fallback_pb_to_pt(client_with_pt):
     assert len(response_json["translatedText"]) > 0
 
 
-def test_portuguese_fallback_pt_to_pb(client_with_pb):
+def test_auto_detect_fallback_pb_to_pt(client_with_pt):
+    response = client_with_pt.post("/translate", data={
+        "q": "Olá mundo",
+        "source": "auto",
+        "target": "en",
+        "format": "text"
+    })
+    
+    response_json = json.loads(response.data)
+    assert response.status_code == 200
+    assert "translatedText" in response_json
+    assert len(response_json["translatedText"]) > 0
+
+
+def test_explicit_language_no_fallback(client_with_pb):
     response = client_with_pb.post("/translate", data={
         "q": "Hello",
         "source": "en",
@@ -52,6 +66,6 @@ def test_portuguese_fallback_pt_to_pb(client_with_pb):
     })
     
     response_json = json.loads(response.data)
-    assert response.status_code == 200
-    assert "translatedText" in response_json
-    assert len(response_json["translatedText"]) > 0
+    assert response.status_code == 400
+    assert "error" in response_json
+