CaptchaHelper: Direct call of `__d()` for each number string

Because of better maintaining `tools.pot`, gettext is called directly for
each number in the array instead of passing a variable. The Cake I18n Shell
better recognizes these strings.

`tools.pot` and `{ces,deu,spa}/LC_MESSAGES/tools.po` have also been
adjusted:

* `cake i18n extract` for generating `tools.pot`
* `msgmerge --no-wrap --update --backup=off $lang/LC_MESSAGES/tools.po tools.pot`
  for each language (`cd cakephp-tools/Locale` before)