A number of friends have done this. With two exceptions, the dogs have all had behavioural issues. Zero recall, escaping from the garden, significant reactivity, resource guarding...
The owners all love them hugely and have invested significant time, energy, and money in behaviourists.
None were fully informed about the potential issues before adopting.
When it works, it works well. When it goes wrong, it can go badly wrong.
If you want to go down this route I would suggest as a bare minimum a rescue that fosters in the UK, so you get an idea of how this dog is in a home rather than in a pound. The two can be hugely different. And research your rescue thoroughly, in particular what backup they give if there are issues.
There are plenty of overseas rescues in UK shelters because they were bought over here, placed in an unsuitable or inexperienced home, and then the rescue would not help when the dog needed to be removed from that home. That's a lousy ending for everyone, including the dog.
To answer your questions:
If they are a couple of years old what about the language.
Not an issue. Assume you'll be training from scratch, anyway, as most of these dogs will have had very limited 'being a good pet' training, so you'll be starting from puppy level regardless.
How do the rescuers know the dogs history.
They don't, by and large. That's why I say go for one who fosters in the UK, so you get as full a picture as possible.