Feature: #97637 - Translate default value of form elements

Description

The TYPO3 form framework now supports translating the defaultValue property of form elements via XLF translation files.

Previously only properties such as label, placeholder, and rendering options could be translated through the form framework's translation mechanism. The defaultValue property was always rendered as-is from the form definition, regardless of the current frontend language.

Now, defaultValue is translated before rendering using the same XLF key conventions already established for other form element properties. The translation is resolved in the following order, with the first match winning:

<form-definition-identifier>.element.<element-identifier>.properties.defaultValue
element.<element-identifier>.properties.defaultValue
element.<element-type>.properties.defaultValue

Example

Given a form element defined as follows:

fileadmin/form_definitions/contact.form.yaml

identifier: contact-form
type: Form
prototypeName: standard
renderables:
  - type: Page
    identifier: page-1
    renderables:
      - type: Text
        identifier: bestDish
        label: 'Best dish?'
        defaultValue: 'Hamburger'
        renderingOptions:
          translation:
            translationFiles:
              - 'EXT:my_extension/Resources/Private/Language/Form/locallang.xlf'

The translation file can now provide a translated default value:

EXT:my_extension/Resources/Private/Language/Form/de.locallang.xlf

<?xml version="1.0" encoding="utf-8"?>
<xliff version="1.2" xmlns="urn:oasis:names:tc:xliff:document:1.2">
    <file source-language="en" target-language="de" datatype="plaintext" original="messages">
        <body>
            <trans-unit id="contact-form.element.bestDish.properties.defaultValue">
                <source>Hamburger</source>
                <target>Kartoffel</target>
            </trans-unit>
        </body>
    </file>
</xliff>

Or, to apply the translation to all Text elements across all forms:

EXT:my_extension/Resources/Private/Language/Form/de.locallang.xlf

<trans-unit id="element.Text.properties.defaultValue">
    <source>Hamburger</source>
    <target>Kartoffel</target>
</trans-unit>

Impact

Form integrators can now provide language-specific default values for form elements. The translation is applied automatically before rendering using the existing translation file configuration in renderingOptions.translation.translationFiles.

Array-based defaultValue properties are intentionally excluded from translation. These occur in multi-value elements such as MultiCheckbox or MultiSelect, where the values are option keys that must match the configured properties.options exactly. Translating them would break the value-to-option mapping. The option labels themselves can be translated via the existing properties.options.[*] translation mechanism.