Need help with encoding

[u/Mindless-Arachnid-74]

I am working on a system where the client will give data in UTF-8 encoding, and hence it can contain many special characters.

But then we send data to a legacy system which only accepts data in ISO-8859-9 encoding, which supports limited special characters that can be represented in single byte.

Now if the client sends any special character which is outside the scope of ISO-8859-9 encoding (like the Em dash), how can I deal with with it and send it to the legacy system.

I don't want to replace those characters with anything like space or "?", as it would make the client data as unreadable in some cases.

How can I deal with it. Any suggestions?