signal-desktop: Allow overriding the spell checker language (#44456)

Thought this could be useful for others as well. Unfortunately it will
also override the UI language.

Example usage:

  environment.systemPackages = with pkgs; [
    (signal-desktop.override {
      spellcheckerLanguage = "de_DE";
    })
  ];

pkgs/applications/networking/instant-messengers/signal-desktop/default.nix

@@ -3,9 +3,24 @@
 , dbus, libX11, xorg, libXi, libXcursor, libXdamage, libXrandr, libXcomposite
 , libXext, libXfixes, libXrender, libXtst, libXScrnSaver, nss, nspr, alsaLib
 , cups, expat, udev
+# Unfortunately this also overwrites the UI language (not just the spell
+# checking language!):
+, hunspellDicts, spellcheckerLanguage ? null # E.g. "de_DE"
+# For a full list of available languages:
+# $ cat pkgs/development/libraries/hunspell/dictionaries.nix | grep "dictFileName =" | awk '{ print $3 }'
 }:
 
 let
+  customLanguageWrapperArgs = (with lib;
+    let
+      # E.g. "de_DE" -> "de-de" (spellcheckerLanguage -> hunspellDict)
+      spellLangComponents = splitString "_" spellcheckerLanguage;
+      hunspellDict = elemAt spellLangComponents 0 + "-" + toLower (elemAt spellLangComponents 1);
+    in if spellcheckerLanguage != null
+      then ''
+        --set HUNSPELL_DICTIONARIES "${hunspellDicts.${hunspellDict}}/share/hunspell" \
+        --set LC_MESSAGES "${spellcheckerLanguage}"''
+      else "");
   rpath = lib.makeLibraryPath [
     alsaLib
     atk
@@ -68,6 +83,7 @@ in stdenv.mkDerivation rec {
              --set-rpath ${rpath}:$out/libexec $out/libexec/signal-desktop
     wrapProgram $out/libexec/signal-desktop \
       --prefix XDG_DATA_DIRS : "${gtk3}/share/gsettings-schemas/${gtk3.name}/" \
+      ${customLanguageWrapperArgs} \
       "''${gappsWrapperArgs[@]}"
 
     # Symlink to bin