All of our translation efforts are handled on Transifex (https://www.transifex.com/nodebb/nodebb/), but in general, yes... fixes and incorrect translations would be handled there, and custom changes specific to your community you'd make to those files directly.
As of version 1.0.0 NodeBB has by default enabled sendmail transport plugin. So it's enough to make available "sendmail" command in your system.
For example on Debian Jessie with postfix installed you can do so with symlink:
ln -s /usr/sbin/sendmail /usr/local/bin/sendmail
and test it like:
echo "Test mail" | sendmail -i -f [email protected][email protected]