Normally, I am developing a standalone module. Each module has it's not resource file under App_LocalResources folder. Once I add the language resource values, DNN automatically increases a new resource file accordingly.
For example :[Your Local Resource File].ascx.resx
once you add additional language the new file will be like this
[Your Local Resource File]-lang.ascx.resx
You have two option to get the resource value through the ascx file or from the code behind depends on your requirement and coding method.
You inherit following class in your code-behind or ascx file DotNetNuke.Services.Localization to achieve your localization objectives