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Hi there,
I understand they have to pass an interview in order to start their training, that would be a starting point.
As far as I know you have to pass a multiplechoice test which is not difficult at all, this was the DOD requirement at my time and for a couple of recent ITAF training programme hold in the US I've been involved with.
That was very basic grammar but really depends on the interview test itself,
you know better than me that knowing a word doesn't mean you are able to put it in a phrase and viceversa.
If they have a decent grammar base you could approach technical language as an excuse to work on their speaking capability which is what they really need.
If they are consceiuous officers wanting to be pilots, which I think they should be, I'm sure they are doing their own reading.
In many years I've personally heard of just one case of failure due to poor language skills in upt, enjpt, supt (the guy didn't even start!).
One good piece of advice though, english could be used to threaten you because it will never be your language.
I remember beeing put under a special monitoring status due to english after I had completed all my T37 solos because i answered working a call at the bottom of the loop rather than during.
Good luck