GDOv7 comes with an own i18n impl.
Trans might not be feature rich, but should be fast.
English is assumed to be spoken by the user, as something is simply not translateable, like the parameter key of an input, like --arg. Not gonna do that! So simple I18n...
echo t('greet_message');
The lang files are php files that return an array.
the strings are given to printf, so you do "%s" in your lang file.
take a look at the core lang file: core_en.php
All translations are merged in a big array, so you can, and should, re-use translations.