It depends

When other co-workers are around, you two should always speak in English.  It is professional if you discuss work; and it is polite when you just chit chat.  

If there are only two of you, I guess you should be able to talk Chinese to each other. 

Refereence the law suit brought by Minado employee for language discrimination.

Manhattan, NY: (Mar-01-07) Minado, a Carle Place restaurant chain, was sued by one of its former employees, Zuo Zhou Lin, following a complaint that was filed with the US Equal Employment Opportunity Commission. The suit accused the restaurant of discriminating against employees based on language after ordering all its Chinese, Korean, and Hispanic employees to speak English only, even on their breaks. Company managers threatened to fire the Chinese employees if they complained, the suit claimed. The office filed suit on Lin's behalf over a year ago in US District in Brooklyn alleging national-origin discrimination. The lawsuit was resolved after Minado agreed to pay $130,000 to Lin as well as pay nearly $4,000 for the EEOC to run legal notices in three Chinese media outlets to reach former employees.