Steps to add a new language :

You want to add Spanish* language?

Fork the project
Clone the project
Copy the file translations/default.php to translations/spanish.php
Replace the English values with Spanish values into the file translations/spanish.php
Add language key in $allowedLangs array into src/Weekend/Translate.php file, line 24 (here 'spanish')
Add language case in method getSentence1() into src/Weekend/Translate.php file, line 56
Commit files
Open a pull request

Thanks :)

*given as example