So, your pitch to them is that you left the game industry to do something else for a couple of years rather than make games in your non-preferred language.
That's not going to go well with "Passion for making and playing games", and while they may want one released title, having released a mobile game a few years earlier and nothing since is not helping you on the professional history.
Also, they were asking for "many years" of rendering programming experience, from your initial post, I don't get the sense that you meet that requirement anyway
You asked if it would be better for you to move back outside of games rather than switch languages. I guess the question is, which is more important to you. To be programming games, or to be programming in C++? Because that's the question as you presented it.