build/windows: Make Installer's Language dialog automatic

Actually, the Language dialog is always displayed, which is not too good since:

1) This is inconsistent: the install mode dialog (to choose user or system)
   appears before using the system lang automatically. Even if Inno fix this:

2) This is tiring: depending on the context, the user can see up to 5 dialogs:
   SmartScreen (1) > Install mode (2) > installer lang (3) > dev warning (4)
   > SetupMutex (5): https://groups.google.com/g/innosetup/c/bkRtnHz2ygg

3) This is redundant: even if only 2 and 3 are displayed, why 2? Inno will
   auto select the sys lang, which the user understand to search GIMP for PC;

4) This is confusing: even the lang dialog message being clear, some users
   genuinely conclude that the selection will carry over to GIMP install like
   the old Adobe CS installer (#5515). !1628 makes more clear that not, but
   reducing confusion even more is a welcome plus considering points above.

So, let's use 'ShowLanguageDialog=auto'. That option ensures that the dialog
will NOT appear if the sys lang have a translation present in the installer,
which matches what every program does and even installers like Inkscape one.

If the lang isn't included in the installer, the dialog WILL appear so the
user can choose one which he is more fluent, being is a nice middle ground.

build/windows/installer/base_gimp3264.iss

@@ -154,6 +154,7 @@ AppSupportURL=https://www.gimp.org/docs/
 
 ;INSTALLER PAGES
 PrivilegesRequiredOverridesAllowed=dialog
+ShowLanguageDialog=auto
 DisableWelcomePage=no
 InfoBeforeFile=gpl+python.rtf
 DisableDirPage=auto