Fork Stirling-PDF and make a new branch out of Main
Then add reference to the language in the navbar by adding a new language entry to the dropdown
https://github.com/Stirling-Tools/Stirling-PDF/blob/main/src/main/resources/templates/fragments/languages.html and add a flag svg file to https://github.com/Stirling-Tools/Stirling-PDF/tree/main/src/main/resources/static/images/flags Any SVG flags are fine, i got most of mine from here If your language isnt represented by a flag just find whichever closely matches it, such as for Arabic i chose Saudi Arabia
For example to add Polish you would add
<a class="dropdown-item lang_dropdown-item" href="" data-language-code="pl_PL">
<img src="images/flags/pl.svg" alt="icon" width="20" height="15"> Polski
</a>The data-language-code is the code used to reference the file in the next step.
Start by copying the existing english property file
Copy and rename it to messages_{your data-language-code here}.properties, in the polish example you would set the name to messages_pl_PL.properties
Then simply translate all property entries within that file and make a PR into main for others to use!
If you do not have a java IDE i am happy to verify the changes worked once you raise PR (but wont be able to verify the translations themselves)