I’m using Xojo 2017r1.1 on a Mac, Safari 10.1.3.
I add in the shown event of my first window [implicit instance is off] this line:
Session.ConfirmMessage = "do you really want to exit this web app?"
When I test it, I go to forum.xojo.com, then open the 127.0.0.1:8080.
When I press the back button, I think I get my question: do you really want to exit this web app?
But I got this question: “soll diese seite wirklich verlassen werden?”
This means the same in German, but this is not my text I used for the session.confirmmessage.
Is this a bug? It ignores what I want to show?
Do you also have the ConfirmMessage property set in the inspector for the Session class?
I added it to the Shown event of the first page.
I don’t have a property for ConfirmMessage in the Inspector.
I found a “Attributes”-table in the inspector of the sessions, there I added “ConfirmMessage” - “test”, but nothing happens.
Ah, sorry. It’s on the App class.
I don’t have the 2017r2. 
I’m still at 2017r1.1, in this app class I only have:
Default WebPage
LaunchMessage
DisconnectMessage
HTML Header
But no ConfirmMessage
It’s not new.
I just looked at the docs and it’s definitely on Session (sorry about that).
I have no idea why your app (or the browser) would be translating it. Have you tried setting your system to another language to see if this box is different? Maybe a different browser?
I downloaded Google Chrome in the newst version and installed it. Here is the same.
But another text:
Translate to englisch it means: “Do you want to leave this Webpage? It is possible, that your changes you have taken will not be saved.”
Another text, not from my code.
I try to switch the language of the Mac from German to English, and test it again.
update: now the Mac is setting to English and restarted.
In safari the web app asks me: “Are you sure you want to leave this page?”
I have no idea why this wouldn’t be working unless the browser makers have finally gotten smart and limited what this dialog can be used for. Because it’s a system dialog, scammers will sometimes put some other ominous message in there.
File a bug report with your sample project and I’ll take a look.