Please ensure that your staff understands what “gluten free” is, it is in my opinion that if you are working in a place that offers “gluten-free” then the ownership should ensure that their staff knows what is and is not “gluten free.
Also staff should be able to understand the prominent language of the area. If you can’t communicate fluently, then there is an issue of misunderstanding and this is where your paying customer will not continue to use your establishment.