How can you handle a foreign customer when you don't know their language, and there's no one around to help?

Never say You Cant....... I agree with Tarek    Body language  and the fact that they might  understand the language but can not speak it, I find alot of times if you  take your time ,listen closlely and pay attention you can figure out what the customer needs 

Use your body language and signs then use the tools around you.
For example, if you figured out they are asking about the price use a calculator. 

You can't.

use the international language ( body language) , second thought you don’t know their language who knows may be they know yours .

Never say u cant understand him .. try in your mother language and also try secound one for exaple arabic & english if he his not understand call someine cancommunicate with him .. at last u must cash him

Use technology which is available online.
Transilate your language to the customer language with google transalater etc.
so that you can understand and convince the customer.
It cannot be100% accurate but some form of communication happening.

You just can not.