Those pythonic nitwits have deprecated getdefaultlocale and not provided a replacement. Blithering idiots.

Fortunately we only really use it on Linux which has no system API
for getting the user's preferred language instead relying on the usual
smorgasboard of under specified, arcane, mutually incompatible env
vars. So just copy the env var parsing code from the deprecated function
and make do. Sigh.

src/calibre/startup.py

@@ -6,10 +6,13 @@ __docformat__ = 'restructuredtext en'
 Perform various initialization tasks.
 '''
 
-import locale, sys, os
+import locale
+import os
+import sys
 
 # Default translation is NOOP
 from polyglot.builtins import builtins
+
 builtins.__dict__['_'] = lambda s: s
 
 # For strings which belong in the translation tables, but which shouldn't be
@@ -19,7 +22,7 @@ builtins.__dict__['__'] = lambda s: s
 # For backwards compat with some third party plugins
 builtins.__dict__['dynamic_property'] = lambda func: func(None)
 
-from calibre.constants import iswindows, ismacos, islinux, DEBUG, isfreebsd
+from calibre.constants import DEBUG, isfreebsd, islinux, ismacos, iswindows
 
 
 def get_debug_executable(headless=False):
@@ -127,7 +130,7 @@ def initialize_calibre():
 
     #
     # Setup translations
-    from calibre.utils.localization import set_translators
+    from calibre.utils.localization import getlangcode_from_envvars, set_translators
 
     set_translators()
 
@@ -141,16 +144,16 @@ def initialize_calibre():
     string
     try:
         locale.setlocale(locale.LC_ALL, '')  # set the locale to the user's default locale
-    except:
-        dl = locale.getdefaultlocale()
+    except Exception:
         try:
+            dl = getlangcode_from_envvars()
             if dl:
-                locale.setlocale(locale.LC_ALL, dl[0])
-        except:
+                locale.setlocale(locale.LC_ALL, dl)
+        except Exception:
             pass
 
     builtins.__dict__['lopen'] = open  # legacy compatibility
-    from calibre.utils.icu import title_case, lower as icu_lower, upper as icu_upper
+    from calibre.utils.icu import lower as icu_lower, title_case, upper as icu_upper
     builtins.__dict__['icu_lower'] = icu_lower
     builtins.__dict__['icu_upper'] = icu_upper
     builtins.__dict__['icu_title'] = title_case
@@ -161,6 +164,7 @@ def initialize_calibre():
         # Name all threads at the OS level created using the threading module, see
         # http://bugs.python.org/issue15500
         import threading
+
         from calibre_extensions import speedup
 
         orig_start = threading.Thread.start

src/calibre/utils/localization.py

@@ -28,6 +28,22 @@ def available_translations():
     return _available_translations
 
 
+default_envvars_for_langcode = ('LANGUAGE', 'LC_ALL', 'LC_CTYPE', 'LC_MESSAGES', 'LANG')
+
+
+def getlangcode_from_envvars(envvars=default_envvars_for_langcode):
+    lookup = os.environ.get
+    for k in envvars:
+        localename = lookup(k)
+        if localename:
+            if k == 'LANGUAGE':
+                localename = localename.split(':')[0]
+            break
+    else:
+        localename = 'C'
+    return locale._parse_localename(localename)[0]
+
+
 def get_system_locale():
     from calibre.constants import ismacos, iswindows
     lang = None
@@ -50,13 +66,12 @@ def get_system_locale():
             traceback.print_exc()
     if lang is None:
         try:
-            envvars = ['LANGUAGE', 'LC_ALL', 'LC_CTYPE', 'LC_MESSAGES', 'LANG']
-            lang = locale.getdefaultlocale(envvars)[0]
+            lang = getlangcode_from_envvars()
 
             # lang is None in two cases: either the environment variable is not
             # set or it's "C". Stop looking for a language in the latter case.
             if lang is None:
-                for var in envvars:
+                for var in default_envvars_for_langcode:
                     if os.environ.get(var) == 'C':
                         lang = 'en_US'
                         break