TYPO3 ext form - How to change the label (header) of the summary page?

I'm using the form extension and that's a part of my yaml file:

  -
    renderingOptions:
      previousButtonLabel: 'Go Back'
      nextButtonLabel: 'Send Message'
    type: SummaryPage
    identifier: summarypage-1
    label: Summary

In the xlf file I can access the buttons like this:

<trans-unit id="BasicContactForm.element.page-1.renderingOptions.nextButtonLabel" xml:space="preserve">
    <source>Send Message</source>
</trans-unit>
<trans-unit id="BasicContactForm.element.summarypage-1.renderingOptions.previousButtonLabel" xml:space="preserve">
    <source>Go Back</source>
</trans-unit>
<trans-unit id="element.BasicContactForm.renderingOptions.submitButtonLabel" xml:space="preserve">
    <source>Send Message</source>
</trans-unit>

But how can I access the label (header) of my summary page? This does not seem to work:

<trans-unit id="element.BasicContactForm.summarypage-1.label" xml:space="preserve">
    <source>Summary1</source>
</trans-unit>    

Neither this:

<trans-unit id="BasicContactForm.element.summarypage-1.label" xml:space="preserve">
    <source>Summary1</source>
</trans-unit>       

You can translate the labels for (summary) pages the same way as you translate the form field labels.

In your case, the following translations keys are possible:

<!-- All elements of type 'SummaryPage' -->
<trans-unit id="element.SummaryPage.properties.label">
    <source>Summary</source>
</trans-unit>

<!-- All elements with the identifier 'summarypage-1' -->
<trans-unit id="element.summarypage-1.properties.label">
    <source>Summary</source>
</trans-unit>

<!-- The single element with the identifier 'summarypage-1' inside the form 'BasicContactForm' -->
<trans-unit id="BasicContactForm.element.summarypage-1.properties.label">
    <source>Summary</source>
</trans-unit>

I wrote a comprehensive tutorial on TYPO3 form translation with many explanations and examples which you can find here: https://www.sebkln.de/en/tutorials/detail/translating-forms-in-the-typo3-form-framework/