diff --git a/pkgs/development/interpreters/python/mk-python-derivation.nix b/pkgs/development/interpreters/python/mk-python-derivation.nix
index 9ce2dedef1b3..58a6cc2c70b6 100644
--- a/pkgs/development/interpreters/python/mk-python-derivation.nix
+++ b/pkgs/development/interpreters/python/mk-python-derivation.nix
@@ -388,7 +388,10 @@ lib.extendMkDerivation {
 
       inherit strictDeps;
 
-      LANG = "${if python.stdenv.hostPlatform.isDarwin then "en_US" else "C"}.UTF-8";
+      env = {
+        LANG = "${if python.stdenv.hostPlatform.isDarwin then "en_US" else "C"}.UTF-8";
+      }
+      // (attrs.env or { });
 
       # Python packages don't have a checkPhase, only an installCheckPhase
       doCheck = false;
diff --git a/pkgs/development/interpreters/python/python2/mk-python-derivation.nix b/pkgs/development/interpreters/python/python2/mk-python-derivation.nix
index 4bdda1506290..817225bcc47c 100644
--- a/pkgs/development/interpreters/python/python2/mk-python-derivation.nix
+++ b/pkgs/development/interpreters/python/python2/mk-python-derivation.nix
@@ -248,7 +248,10 @@ let
 
         inherit strictDeps;
 
-        LANG = "${if python.stdenv.hostPlatform.isDarwin then "en_US" else "C"}.UTF-8";
+        env = {
+          LANG = "${if python.stdenv.hostPlatform.isDarwin then "en_US" else "C"}.UTF-8";
+        }
+        // (attrs.env or { });
 
         # Python packages don't have a checkPhase, only an installCheckPhase
         doCheck = false;