They've made CJK characters (including fullwidth characters of any kind, without exception for Latin letters or Arabic numerals) count as 2 characters. In a mixed alphanumeric/CJK tweet, the CJK part will simply count doubly towards your character limit.
Interesting... of course 70 characters of Mandarin would be the equivalent of over 400 characters of English. I suppose that plus goog-Translate would be another even less readable way to avoid the limit... since Chinese language speakers by and large just use WeChat.
