Silverlight can use resource to support multiple languages. We do not talk about the regular steps which how to use resource files.
Here we have to know there is additional step to do in Silverlight 4 or 5, at least in Visual Studio 2010 (We did not try Visual Studio 2012 and Visual Studio 2013):
1: Right click Silverlight project and choose “Unload project”;
2: Right click Silverlight project again and choose “Edit [your Silverlight project name].csproj”
3: find <SupportedCultures> attribute and add more languages, use ‘;’ to separate each culture.