I wanted to have Ianto say something to Jack in his own language, but I think actually Jack would find it distinctly upsetting. Not only is it a reminder of his past, but it's something that someone in the 21st century shouldn't know, and most frighteningly it means he's let his guard down.

I can see, at some point if Ianto survives long enough, Jack teaching him words in his language, but Ianto's too wise to let on to Jack that he's slipping :D