how to choose appropriate resource file

[mgworld]

I'm creating a multi-language website. I have resource files "lang-en" , "lang-fr" , "lang-fa".now i want to use it like:\[code\]<a href="http://stackoverflow.com/questions/12732607/#"><%= Resources.SelectedLanguage.signup %></a>\[/code\]but I can only choose like:\[code\]<a href="http://stackoverflow.com/questions/12732607/#"><% Resources.lang-en.signup %></a><a href="http://stackoverflow.com/questions/12732607/#"><% Resources.lang-fa.signup %></a><a href="http://stackoverflow.com/questions/12732607/#"><% Resources.lang-fr.signup %></a>\[/code\]how can I use a variable to select the language?