First, you should not modify mojoportal code.
Second, I think it is a bad idea to make assumptions based on the browser language setting. Maybe my browser is set to French but I still want to read your English site and you won't let me?
I think it would be better to make a dropdown inside a user control that you can put in your layout.master file. The user can choose the language and you make the dropdown do autopostback, on pstback you set a cookie with the user preference and then redirect to the Request.RawUrl to get out of postback.
To redirect the user based on the cookie, you could implement a custom HttpModule which can be plugged in from Web.config and has the same events as Global.asax.cs
In Application_BeginRequest event in your custom HttpModule you can detect the cookie and redirect based on the cookie value.
Hope it helps,