But it doesn’t have to be up to modern standards, and certainly doesn’t have to be with modern materials. Get the local cooper to make your pipes and reservoir with pitch-sealed wood. Or make it out of stone, or cast copper, or whatever they use to store water anyway. If it’s Roman or post-Roman, they’ve already had some experience with running water anyway, that wouldn’t be the impressive bit.

Threading and such is mostly useful for mass-manufacturing standard pipes and using it everywhere, but at least at first you’d just be doing it for a rich/powerful person or two, where you could do something labour-intensive and unscalable.

I’m not saying that you would get a perfect, modern system straight away, but if you can convince the people to give you the benefit of the doubt through a prototype or two, you could make something that works well enough. That would be what I’d be concerned about, even if you can magic away the language barrier, they’d likely just think you’re mad.