Schlep makes a really valid point which has been made by others too, this is more about manners than language. Of course it is fine to answer a customer's question in the language they ask in but it is not ok to use language in a way that excludes or alienates others either when working or on breaks
I worked abroad for a few years in a country that spoke English and another language which is very different to anything we learned at school and I was pretty rubbish at it but I tried, I tried really hard, Equally my colleagues, for whom English was a second language, tried hard too. That's how manners work and it is sad that we have to have these conversations, accusations of racism, xenophobia, etc because people forget them