How to change encoding in Mail in OS X Mavericks?
Solution 1:
It seems you're not the only one with this problem.
I found a KB article on Apple's website that clearly specifies that Message > Text Encoding should be available in the Mavericks version of Mail:
http://support.apple.com/kb/PH14955
It says "Mavericks" in the title and is current as of Nov 6, 2013.
I've filed a bug report letting the OS X team know.
Solution 2:
Have the same problem. Tried the thread from Alex comments (here), seems can solve my problem. But Apple should bring back the Text Encoding option.