Put all the language files on the langs folder, they are all PHP files. On every file define the same variables but with the respective translation. For exemple, in the en-us.php you can define the variable $helloWorld:
$helloWorld = "Hello World";
So in the pt-br.php, you do the translation of the same variable:
$helloWorld = "Olá Mundo";
First of all, change the primary language in the line 6 of app.php:
const PRIMARY_LANG = 'pt-br';
This string have to match the language filename (without the extension). If the script find a primary language, then the static files will be generated in the root of the output folder (/output/), instead of the respective lang folder (e.g. "/output/en-us").
Put all the files to be processed in the files folder, every PHP file will be converted to the respective HTML file on the output folder. To print something that have to be translated, use PHP on the field:
<h1><?= $helloWorld ?></h1>
The pre-processor will generate the file with every language on the respective folder. The files that aren't PHP, will remains intact.
Simply run the app.php in a browser or command line:
php app.php