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.
One learns one's language from one's peers.