I'm starting to learn Swedish and I came across the expression "gå på ett visst ställe" as a way to say "to go to the toilet". I wonder if this is a common expression in daily Swedish and if everybody would understand what I mean if I use it.
To me it's neither common nor immediately self-explanatory without context.
After you explained it it makes more sense, and the way I make sense of it is that "gå på toaletten" is a set phrase and what we're doing is just substituting "toaletten" for "ett visst ställe". Then it makes sense. But of course that necessitates some sort of context that implies that that's what we're talking about. If I knew that you had stomach problems for example, and we were having dinner or something, then perhaps you'd say that and I'd understand. But if we'd had a long conversation about going to the movies then perhaps I'd think you were talking about a specific movie theater instead.
I don´t know how common it is in the Scandinavian languages today, and as such, context may be necessary. That said, my Danish mother has frequently used it, tongue in cheek, however. It was a euphemism from a time when going to the bathroom/toilet was not quite the proper thing to say, jeg skal et vist sted hen.