Can I be fired for speaking English?

There is no simple answer. As a general matter, an employer may not discriminate against an employee due to national origin, and discrminating against them in employment due to their language is generally held to be equivalent to national origin discrimination, so typically, you could not be fired for only speaking fluent english. But there are exceptions, if the employer can establish that speaking another langauge is a critical part of the job--for example, a social worker or program coordinatory serving a heavily hispanic community can legally be required to be bilingual in Spanish as well as speaking English. It is possible that in this situation, IF the customers of the market are heavily made of native russian speakers, that speaking russian would be a legitimate part of the job. There is no harm or cost to you in contacting either the federal EEOC or yoru state's equal/civil rights agency to file a complaint: if there is something to the complaint, the agency may pursue it for you, or else recommend that you hire a private attorney to pursue it; and if there is nothing to it, the agency will so advise you but it will cost you nothing. Therefore, you should reach out to one or both agencies.