This would entirely depend on your approach. In a classic mvc application you could access the pages RouteValues, copy them and add another value such as lang=en and call @Url.Route(routeValues) or similiar.
Alternatively you could get the full page url (think it was Request.Url) and do a hard coded replace via javascript
(ex. var hostName = 'http://www.mysite.com'; window.location.href = window.location.pathname.replace(/hostName/i, hostName + '/en')) but will take some work making it work in all scenarios. Honestly, I'm not sure how to do this with Kooboo ^.^.