Responses to the post included threats to never visit the cafe again.
"This is terrible... Never going there again," one said, while others described the sign as "gross" and "racist".
Another quipped that it "Sounds like they have clowns in management there".
Circus Circus Cafe's general manager Nixon Shrhan told the Daily Mail that management hasn't had any issues with the requirement and it was designed to aid communication between staff.
"Everyone is from different countries and ethnicities, the reason is to know what everyone is saying," he said.
"We just want everyone to speak the same language at work and communicate better."
New Zealand recognises three official languages: English, te reo Māori and New Zealand Sign Language.
On its website, the New Zealand Human Rights Commission states that: "The right to use one's own language is an internationally recognised human right, which is mentioned in a range of human rights treaties and declarations.
"It would be hard for an employer to justify a total 'English only' policy when the reason for that was to create 'workplace harmony' or because it was part of 'company culture'."
However, the commission also points out that such a policy could be appropriate if "an employer has a valid reason for it" such as health and safety.
