allow using a language that is not installed in the platform for existing nodes

in hosting_site.module

change the language test to

if (!array_key_exists($node->site_language, hosting_get_profile_languages($node->profile, $node->platform)) && !$node->nid ) {
form_set_error('site_language', t('Please fill in a valid language'));
}

otherwise you cannot edit imported sites in languages not present on the platform