casper-bottom/14locales should check if locale isn't already generated - generation uses a lot of memory and slowdowns startup

Currently en_US.UTF-8 locale is generated every time during liveCD/liveUSB startup :( See these lines in less /var/log/casper.log :

Begin: Setting up locales... ... Generating locales...
  en_US.UTF-8... done
Generation complete.
done.

Locale generation uses a lot of memory and slowdowns startup, especially on slower computers.
en_US.UTF-8 locale should be already generated in filesystem.squashfs and other locales should be generated during liveCD/liveUSB startup only if user chooses another language in syslinux.

It seems Ubuntu 12.04 and newer releases already contains en_US and other en_* locales in filesystem.squashfs (they are generated during installation of language-pack-en-base package), but script casper-bottom/14locales doesn't check if locale isn't already generated :(
I think command "localedef --list-archive" should be used for checking if needed locale isn't already generated or maybe we need to find why /usr/sbin/locale-gen command, used in 14locales script, doesn't detect that en_US .UTF-8 is already generated?

I can write a patch for checking if needed locale isn't already generated - this is just one line if using "localedef --list-archive" ;)