I guess you have to define fluency. Is it when you can think and respond in another language? I have known people who were fluent in 3 or 4 languages (while I have enough trouble keeping just my second language from withering away)
A lot of people know romance languages which have some basic similarities. When they are in a position to use one of them, it improves with use.
Where I work, everyone speaks English except when they are talking to their spouse on the phone. THEN I hear a lot of other languages.
If you're fluent in Chinese but rusty; put it on your resume! If you ended up in China for a month I bet you'd do better than someone without that fluency!