Simple: it just is.
Every customer is a potential buyer. But how can you know when a random customer comes to your shop and doesn’t speak your language?
You don’t. That’s why you have to study different languages.
Obviously you make a good impression when you can speak the customer’s language if he or she is a foreigner.
In worst case the customer knows only his or her own language, and you can’t speak it, so you’re incapable to help the customer.
We don’t want this to happen, don’t we.
But in the first place, this will help you get the job. After it, you can start using your skills.