Looking at the DNN code is always a good idea since it does the same you need. In this case this is the code we use in the language selector to change the current language:

        Private Sub selectCulture_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles selectCulture.SelectedIndexChanged

            ' Store selected language in cookie
            Localization.SetLanguage(selectCulture.SelectedItem.Value)

            'Redirect to same page to update all controls for newly selected culture
            Response.Redirect(Request.RawUrl, True)

        End Sub

Notice that you have to reload the page (redirect) in order for the language to take effect. Hope this helps.

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Vicenç Masanas
Banyoles, Girona - Spain

   

Depending on the version you are using this is a problem with cached modules. If you have a module that needs to render different content while in the view presentation you should disable caching on it (you'll find this option under module settings, set the cache time to 0).
In latest version I think this has already been solved.

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Vicenç Masanas
Banyoles, Girona - Spain