According to the MSDN documentation here, The basename argument specifies "The root name of the resource file without its extension but including any fully qualified namespace name. For example, the root name for the resource file named "MyApplication.MyResource.en-US.resources" is "MyApplication.MyResource"."
The ResourceManager will automatically try to retrieve the values for the current UI culture. If you want to use a specific language, you'll need to set the current UI culture to the language you wish to use.