so how can they improve their English? Especially when they work long hours
By working with English-speakers and using English in the workplace might help.
If any of my Polish colleagues are interested in learning English, I do try to help.
The result? I learn a bit of Polish and they are given the opportunity by me to learn a bit of English.
One colleague I've worked with for about five months has improved a huge amount in that time. He does two jobs, but he is young (and therefore quite malleable) and very keen to learn.
Queen's english
One learns one's language from one's peers.