I am having an issue with non-Latin characters. For example, if I go to the link below and type in "ü" and press submit, İ get %C3%BC instead of that letter. I have tried to use
content="text/html; charset=utf-8 "
You can't really avoid the issue; decoding of such characters needs to happen server-side. In PHP the urldecode function is used for this, so urldecode('%C3%BC') returns 'ü'. This behaviour is part of the way that forms work - if you look at the code for the Stack Overflow form that I'm using to submit this answer:
<form id="post-form" action="/questions/37262431/answer/submit" method="post" class="post-form">
I'm able to use the letter ü, but the form doesn't contain any extra attributes, because the server is decoding the '%C3%BC' before returning 'ü' to the client.