Removed direct sprintf() usage from _t() calls

Parameterized strings are easier to understand,
and more fail-proof, don't fatal out when not enough sprintf() args
BUG Graceful handling of sprintf with too few params in i18n::_t()

Originally discovered as a problem with the 'GridFieldDetailForm.Saved' in nl.yml
Added docs about which branch to choose
BUG: Confirmed Password Field now copies attributes to child fields.