@Colin Bell Yes, there is only one language pair for English to French. So, if this pair is selected for one project another project with the same category and pair needs to have a different label to differentiate the category. This may not be ideal in your case if you are building your project for the same purpose or a customer.
You can however try to create projects with English-French in different workspaces with same category or labels and then use the corresponding category ids as parameters when you call the API because a CategoryID is created by concatenating the WorkspaceID, project label, and category code. I hope this helps as a workaround.