Fix locale

os/common.nix

@@ -31,6 +31,13 @@
     git # Needed for all the fetchFromGit in this repo on nixos-rebuild
   ];
 
+  i18n = {
+    defaultLocale = "en_GB.UTF-8";
+    extraLocaleSettings = {
+      LC_TIME = "en_DK.UTF-8"; # Should give YYYY-MM-DD but doesn't work for browser & Thunderbird :(
+    };
+  };
+
   nix.settings = {
     experimental-features = [ "nix-command" "flakes" ];
     warn-dirty = false;