WordPress page with non-English text

I have a simple WordPress page that takes input from users and store into a database. I am able to get the values and update the database properly. However, whenever I try to enter some non-English text, it gets converted to something like ‘?????????’. But the issue is only with my page. If I use the WordPress comments form, the text is stored in the database correctly.

My complete code is available at https://pastebin.com/MDLJBXpQ

Please help