Esto si creo que te podría servir:

http://www.snowcovered.com/snowcovered2/Default.aspx?tabid=242&PackageID=10394

(es gratuito) Dice lo siguiente:

Assuming you want to offer your website in english and in french. You define two root pages ("en" and "fr"), one for each language, and under each of these pages, you create the site structure.

Initial redirection

When a user accesses www.yourdomain.com, the extensions will do the following:

- detect supported language of the current portal by looking at root pages (in this case, en and fr)
- detect the user's browser languages and keep the first one that is supported by the current portal
- if it can't find a suitable preferred language, use the portal default language
- redirect the user to the first child page of the selected language (it will redirect for instance to "en/Home.aspx")

Navigation Menus

By default, DNN will show the root language pages ("en" and "fr") in root menus. To create a page hierarchy that corresponds to current language (pages under the current root language page),  the extension uses custom navigation providers.

The extensions comes with support for these navigation controls:

- DNNMenu
- SolpartMenu
- Breadcrumb

If using another menu, you will need to adjust the navigation provider for your specific menu.

Por fin!!!!! ya está!!! en este trozo de código

                If UserId <> -1 Then
                    If User.Profile.PreferredLocale <> CultureInfo.CurrentCulture.Name Then
                        _RedirectURL = UrlUtils.replaceQSParam(_RedirectURL, "language", User.Profile.PreferredLocale)
                    End If
                End If

En negrita podeis ver que pregunta si el lenguaje del usuario es distinto que el lenguaje actual, si es asi cambia el lenguaje al preferido del usuario, deshablitando estas líneas consigo lo que yo quería, que el lenguage se quede en el que esté independientemente del lenguaje preferido del usuario.

Gracias a todos por vuestra ayuda.