Thank you for posting.
FileMaker Server will generally ask you to select a language when you are installing it. What operating system is the server? What is the server's default language/region set to?
Have you tried using multiple web browsers to test? If you create a new file on that server, does the text display? Some component in the system may not be properly interpreting the character encoding. We want to try to narrow it down to either FileMaker Server, PHP, the OS settings, or a particular web browser.
If you would like to send a clone or sample file, I can test it here. I have sent you a private message with instructions for sending us your file.