2). I really don't understand why we should use Twig : Smarty is older, why using yet another template engine ? Or I've got a better idea : we could use the template engine used in phpBB2 : he's older !
I'm agree that standardization would be just fine(c), and I like the Twig syntax, but I think the "YATL, we should use Twig" argument is not relevant.
2). See: http://twig.sensiolabs.org/ (Why yet another template engine?)
3). Personal preference I guess but I hate gettext. I much prefer the pattern of using labels in your template (e.g. menu.item1) and then looking up that label in the desired language file e.g. messages.en.yml. And I've never had a problem with translators working with YAML files (i.e. the syntax is easy for them to grok)