The mail and page encoding must match.
Short answer: Content-encoding: utf-8.
Long answer, let the document containing form have Unicode encoding- that takes care of almost all characters, and if it's sent via oridinary "mailto:" protocol, that should be automatically recognized and converted by mail clients.
Other thing is that if the form is submitted to a script that mails it content, it's even easier:
In PHP, for example you have
and that's it, it takes care about encoding
mail("email@example.com", "Form data", $formcontent,