What is the exact version and hotfix you are using? A similar bug was fixed in hotfix no.62:
Localization - Incorrect localization of form properties on MVC sites
Localizing a form's 'After the form is submitted' or 'Submit button' text using resource strings (on the 'General' tab of the form's editing interface) did not work correctly for forms displayed on MVC sites using the 'Form' widget. The live site always displayed the English version of the text instead of using the current page's culture.
To fix this issue, please download and apply the latest hotfix. Follow the instructions and do not forget to update the NuGet packages in the MVC project too.