This is a known issue. Any user text that appears in CoreGui does not get translated right now. That includes the legacy dialog system, among other things.
Right now I’m working on something that will allow us to call the auto-translator in scripts, which will let us fix this issue. It might take a few months to ship proper, though.
I would also suggest not using that ancient dialog system as well. There’s nothing there that you can’t do yourself, and any other dialog system would not use CoreGui and would easily translate today, without needing to wait for my fixes.
It’s not hard to do the same thing yourself. @Davidii wrote a pretty good one while he was an intern here. Consider giving that a try and customizing it a bit.
Otherwise we’ll get the fix shipped as soon as we can.