Just another thing that springs to mind is my attempts at learning French at school .. 2/3 periods x 40 mins per week over 5 years !! Came out not being able to hold a decent conversation in French in France !!
Although a 'skimming' is possible to learn whilst still in their own country, not really possible until you actually live with 'us natives' .. different alphabet too.
Think rather a commitment of some sort that once here they will undertake classes and groups to learn our language, otherwise, back out again. Mind you, who will be the judge as to whether their English is good enough to stay. After all, have to learn all the special sayings we have as depicted in one of our threads on Lancashire sayings ..
Can you imagine teaching Asians 'E-by-gum'
Sorry being frivolous.