If an employee doesn't like an employer's English-only policy, the employee is perfectly free to seek employment elsewhere. Likewise, if an employer does not want his employees to speak Spanish, he should be perfectly free to let them go if they do so.
I feel the same way whether the employer wants an English-only or a Spanish-only policy. This is not something the government needs to meddle with.
I can name one major US city where it is almost impossible to get a job if you don’t speak Spanish...El Paso, Tx.