Yes, you can indeed translate the Signup form headings, labels, and messages text to another language. For that, please do the following:

1. From the Back in Stock - Restock Alerts.

2. Go to Customize-> Signup Form-->Product page button-->button appearance-->Signup form labels.
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Here you can translate manually all the headings, labels, and messages text on the 'Form Labels' and the 'Confirmation Messages' sections.

3. Click on the 'Save' button.

Note: As of now, the Signup form can be displayed in only one language per store. We don't support auto language translations yet. We’re working to get integrations with multi-language apps though.