On your form you wish to localize for a language change the property Localizable to true.
Then change the Language property to the language you wish to set up a new resource file for.
Change all the text on the form to the new language. This will create an additional resx file for each language you change the Language Property too.
Once you finish you can change to another language for another translated form resource file or switch back to default.
Now when the computer is run on a machine set up to use a different language it will look for a resx file to match and switch all the controls to use the text provided in the resource file.
You can also test this by changing the CurrentUICulture on the CurrentThread.