Re: Introducing a foreign [created] language
That'll teach me to read things properly before jumping to a conclusion -- you said you'd got this language, lo and behold Iris and Terry hear someone coughing, and I think what a brilliant way to show an alien language being spoken... ahem. Sorry!
Anyway, if Keiro would say this, whatever it is, and if Iris and Terry wouldn't be able to interpret it, then I think it's right to introduce it in this way. For my taste I don't think you should have too much of it at any one time, otherwise you might find your readers start jumping bits, but the odd sentence is fine.
But, please, do not have Keiro and his fellows speak impeccable English, including words native speakers might find difficult, and then lapse back into their own language for little words which are easily translatable-- the kind of thing that has a Frenchman saying 'Mon ami' to remind everyone he is French!
Just one other little point. I don't know about you, but if some maniac leapt out of an alley, yanked on my arm and shouted gibberish in my face, I would not be thinking 'Ooh, I've heard someone else say that'. I'd be pulling myself away, screaming blue murder and possibly landing one on the assailant. I'd suggest either tone Keiro down or bump up Terry's reaction.