If you are using Extradium in a multilingual farm, you will notice that the sign in form displays an additional drop-down language list that displays the languages currently enabled on your SharePoint site.
However, if you try to select another language, you may notice that the UI language does not work. You will need to perform the following additional configuration steps in order for the language switch to work:
1. Open the web.config file of the web application zone where you enabled Extradium
2. Locate the <system.webServer><modules> node and add the following line after all the listed modules (the last one available with SharePoint Foundation is <add name="SPApplicationAuthentication" type="Microsoft.SharePoint.IdentityModel.SPApplicationAuthenticationModule, Microsoft.SharePoint.IdentityModel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />):
<add name="ExtradiumMUISwitcher" type="RioLinx.Extradium.Utilities.MUISwitcherModule, RioLinx.Extradium.Utilities, Version=3.0.0.0, Culture=neutral, PublicKeyToken=ee2f606d34db8cb1" />
3. Save your web.config file
4. Browse to the sign in page. If you have enabled more than one language on your site, you should be able to conveniently switch from one language to another. However, note that Extradium is currently only supported for the following languages: English, German, French, Spanish, Portuguese and Dutch. If you have enabled any other language on your site, the user interface of the Extradium sign in page will default back to English.