My Take: If she doesn't speak any English she's almost certainly working not of her own will.
I think you should report to the crimestoppers and save others the trouble.
I don't agree at all, loads of foreign WGs cant speak the lingo, it doesn't mean they are almost certainly working not of their own free will in my opinion. Some come here not speaking the lingo to make good money which they cant make at home, its that simple. They cant possibly afford to rent a premises so work for a pimp instead who provides it and lays the punters on for a cut of the WGs fees.
It is of course up to the OP if he decides he thought the WG wasn't working of her own free will and what he does about it. Sadly there are some WGs who wont be working of their own free will and its for punters to use there best judgement what to do about that if they think they have met such a woman.
There are 2 types of pimps, 1 is an evil cunt who forces woman to be WGs, such scum should be caught and get a mandatory life sentence in my book. Then there are the pimps who take an agreed cut of a WGs fees and may provide the premises, a WGs biggest expense by far which some WGs cant afford to provide themselves. I have had a number of regulars from EE countries and their logic was clear to them, they didn't want to rent a premises themselves and couldn't provide the references required anyway so said they actively looked through a fellow country WG for a pimp to work for. They agreed to hand over an agreed cut and made great money compared to what they could by staying at home. Also loads of EE WGs come here work for some weeks then go back home for a few weeks, having no ties like renting a premises here suits them. This scenario rightly or wrongly has been going on for many many years of course.
IMO its wise not to jump to conclusions but that's for punters to decide, some WGs/pimps are just bad WGs looking to rip/con/fuck punters over, they just have bad attitudes.