Currently, the form captcha control is not localizable. In order to localize the fields you need to implement a custom solution by extending the default form captcha control and plugin your custom logic there.
You may take a look at the following articles for more details:
Here is also a sample code as we have suggested in the support ticket you have opened illustrating how you can add your custom logic in the custom form captcha control and change the field values:
CustomFormsCaptcha : FormCaptcha
"This is the custom link"
"This is custom label"
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