Editing an existing language file in WorkflowGen is similar to the method to add a new language (see How to support / create a new Language).
The same caveat applies that software updates may overwrite any changes made to your language resource files. To avoid this, back up the resource files prior to an update, perform the update, and reapply the changes afterwards.
Special note for the English US language
English US (en-US) is the primary language of WorkflowGen, and as such, does not have a language-denominated resource file associated to it in the same manner as other languages. For example, English Canada (en-CA) has the Advantys.Workflow.Web.UI.Portal.en-CA.resx resource file for the Portal module, whereas the default core language for WorkflowGen (en-US) is maintained in the Advantys.Workflow.Web.UI.Portal.resx resource file.
Attempting to modify the Advantys.Workflow.Web.UI.Portal.resx file, however, will not function as expected. Instead, to create changes to the English US language, copy Advantys.Workflow.Web.UI.Portal.resx (and other resource files as described in the link above) to a new Advantys.Workflow.Web.UI.Portal.en-US.resx file and modify this new file. Changes to labels will then appear as expected.