Do not add a language fallback by the stripping country code (#9635)

Author: alvinhochun

CHANGES.rst

@@ -119,6 +119,8 @@ Bugs fixed
   titles tend to rank higher than subsection titles. In addition, boost matches
   on the name of programming domain objects relative to title/subtitle matches.
   Patch by James Addison and Will Lachance.
+* #9634: Do not add a fallback language by stripping the country code.
+  Patch by Alvin Wong.
 
 Testing
 -------

sphinx/locale/__init__.py

@@ -114,11 +114,7 @@ def init(
         translator = None
 
     if language:
-        if '_' in language:
-            # for language having country code (like "de_AT")
-            languages: list[str] | None = [language, language.split('_')[0]]
-        else:
-            languages = [language]
+        languages: list[str] | None = [language]
     else:
         languages = None