When you are installing the Checkout module with spanish language in a Gallery 2.2.6 site, you can obtain a "ERROR_PLATFORM_FAILURE" caused by the "es.mo" file (missing?).
I've apparently solved this issue copying the file /modules/checkout/locale/es/LC_MESSAGES/modules_checkout.mo as /modules/checkout/po/es.mo
To do this in a command line:
cp -p /modules/checkout/locale/es/LC_MESSAGES/modules_checkout.mo /modules/checkout/po/es.mo
..and then refresh setup page or repeat the module installation.