It depends on your email application. If you are using Hotmail and Internet Explorer 8+, then to read an email that was written in another language, you can use the built in
Accelerator. Highlight text, click on the Accelerator icon, and then hover over Translate with Bing. A box will appear with the translation. To write the reply, you can use
http://bing.translate.com and then copy / paste your answer.
If you use Outlook 2010, then you can use the Translate feature in it.
--Lora