Textbox sends me question marks

[J-T]

I have a textbox on my site which users can enters thier commecnts and when
they hit the send button ,and email is sent to me which contains the
comment in its body.it is fine with English inputs.However when thier write
in their my own native language ,I get an email with ? instead of my
langugae letters! Shall I do any sort of localization in my asp.net app? or
I can set UTP-8 for my textboxex? the problem is that I don;t know wehter
the user writes in English or my own language.

Any help would be appriciated.

Thanks