This is a Django repository issue but the current directory doesn't contain Django code. I need to understand the issue based on the problem description and create the appropriate patch.

Based on the issue:
1. The error `translation.E004` is raised when `LANGUAGE_CODE` is set to a sublanguage (like "de-at") that isn't in `LANGUAGES`, even though the base language ("de") is available
2. According to Django documentation, sublanguages should fall back to base languages
3. This was introduced as a regression in commit `4400d8296d268f5a8523cd02ddc33b12219b2535`

The fix should modify the translation check to also allow sublanguages when their base language is available. Let me create the patch based on Django's translation checks module structure:

Based on my knowledge of Django's codebase, the translation system checks are in `django/core/checks/translation.py`. The `E004` check validates that `LANGUAGE_CODE` is in `LANGUAGES`. The fix needs to also check if the base language (e.g., "de" from "de-at") is available when the exact sublanguage isn't.

Here's the patch:

--- a/django/core/checks/translation.py
+++ b/django/core/checks/translation.py
@@ -55,7 +55,12 @@ def check_language_settings_consistent(app_configs, **kwargs):
     """Error if language settings are not consistent with each other."""
     available_tags = {i for i, _ in settings.LANGUAGES} | {'en-us'}
     messages = []
-    if settings.LANGUAGE_CODE not in available_tags:
+    # Check if LANGUAGE_CODE is in available tags, or if its base language is
+    # available (e.g., 'de' for 'de-at'). This matches Django's language
+    # fallback behavior documented in the translation docs.
+    language_code = settings.LANGUAGE_CODE
+    base_language = language_code.split('-')[0]
+    if language_code not in available_tags and base_language not in available_tags:
         messages.append(Error(
             'You have provided a value for the LANGUAGE_CODE setting that is not in '
             'the LANGUAGES setting.',