Customizing invoice template

Currently using a different software but looking at invoice ninja, as in my preliminary testing the Khmer language printing worked great (which is not the case of my current system, forcing to use spreadsheets for customer documents…).
So I of course need a custom invoice template as both English and Khmer are needed in the same document.
I have made a few attempts at customizing the template and they all end up with either error 500 or worse. invoice ninja is currently all newly installed and updated on a Cpanel account. Looking at the current documentation didn’t help much.
I understand it is a very vague start but I would really like to make it happen. So please do not hesitate with questions or tips as I am definitely willing to try and make it work.

Thank you.