diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 855f34e8a..a9cd778bc 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -1018,23 +1018,75 @@ class Coder:
             ]
         self.cur_messages = []
 
-    def get_user_language(self):
-        if self.chat_language:
-            return self.chat_language
+    def normalize_language(self, lang_code):
+        """
+        Convert a locale code such as ``en_US`` or ``fr`` into a readable
+        language name (e.g. ``English`` or ``French``).  If Babel is
+        available it is used for reliable conversion; otherwise a small
+        built-in fallback map handles common languages.
+        """
+        if not lang_code:
+            return None
 
+        # Probably already a language name
+        if (
+            len(lang_code) > 3
+            and "_" not in lang_code
+            and "-" not in lang_code
+            and lang_code[0].isupper()
+        ):
+            return lang_code
+
+        # Preferred: Babel
+        if Locale is not None:
+            try:
+                loc = Locale.parse(lang_code.replace("-", "_"))
+                return loc.get_display_name("en").capitalize()
+            except Exception:
+                pass  # Fall back to manual mapping
+
+        # Simple fallback for common languages
+        fallback = {
+            "en": "English",
+            "fr": "French",
+            "es": "Spanish",
+            "de": "German",
+            "it": "Italian",
+            "pt": "Portuguese",
+            "zh": "Chinese",
+            "ja": "Japanese",
+            "ko": "Korean",
+            "ru": "Russian",
+        }
+        return fallback.get(lang_code.split("_")[0].lower(), lang_code)
+
+    def get_user_language(self):
+        """
+        Detect the user's language preference and return a human-readable
+        language name such as ``English``. Detection order:
+
+        1. ``self.chat_language`` if explicitly set
+        2. ``locale.getlocale()``
+        3. ``LANG`` / ``LANGUAGE`` / ``LC_ALL`` / ``LC_MESSAGES`` environment variables
+        """
+        # Explicit override
+        if self.chat_language:
+            return self.normalize_language(self.chat_language)
+
+        # System locale
         try:
             lang = locale.getlocale()[0]
             if lang:
-                return lang  # Return the full language code, including country
+                return self.normalize_language(lang)
         except Exception:
-            pass
+            pass  # pragma: no cover
 
-        for env_var in ["LANG", "LANGUAGE", "LC_ALL", "LC_MESSAGES"]:
+        # Environment variables
+        for env_var in ("LANG", "LANGUAGE", "LC_ALL", "LC_MESSAGES"):
             lang = os.environ.get(env_var)
             if lang:
-                return lang.split(".")[
-                    0
-                ]  # Return language and country, but remove encoding if present
+                lang = lang.split(".")[0]  # Strip encoding if present
+                return self.normalize_language(lang)
 
         return None