Description

This PR fixes the XLIFF export format to comply with the XLIFF specification. Previously, the <source> tags contained translation keys instead of the default locale text, making the exported files non-compliant and difficult to use with translation management software.

What it does and why it's needed

The XLIFF specification requires that <source> tags contain the original text to be translated (from the default locale), not the translation key. This fix modifies:

1. XliffFileDumper - Updated to use metadata['source'] when available for both XLIFF 1.2 and 2.0 formats
2. XliffFileLoader - Updated to store the source text in metadata when loading XLIFF 2.0 files
3. Added comprehensive unit tests to verify the behavior

Before / After Comparison

Before:

<trans-unit id="0umDnnC" resname="navbar.home">
  <source>navbar.home</source>
  <target>Accueil</target>
</trans-unit>

After (with metadata):

<trans-unit id="0umDnnC" resname="navbar.home">
  <source>Home</source>
  <target>Accueil</target>
</trans-unit>

How it works

When a MessageCatalogue has metadata['source'] set for a translation key:

$catalogue = new MessageCatalogue('fr');
$catalogue->set('navbar.home', 'Accueil', 'messages');
$catalogue->setMetadata('navbar.home', ['source' => 'Home'], 'messages');

$dumper = new XliffFileDumper();
$xliff = $dumper->formatCatalogue($catalogue, 'messages', ['default_locale' => 'en']);
// Generates: <source>Home</source> instead of <source>navbar.home</source>

Backward Compatibility

✅ Fully backward compatible - If metadata['source'] is not set, the dumper falls back to using the translation key as before, ensuring existing workflows continue to work.

Tests Added

• testFormatCatalogueWithSourceMetadata() - Verifies XLIFF 1.2 uses source metadata
• testFormatCatalogueXliff2WithSourceMetadata() - Verifies XLIFF 2.0 uses source metadata
• testFormatCatalogueWithoutSourceMetadataFallsBackToKey() - Ensures backward compatibility
• testLoadVersion2StoresSourceInMetadata() - Verifies loader stores source in metadata

All tests pass successfully.

Benefits

This fix enables:

• XLIFF compliance - Files now conform to the XLIFF specification
• Better translation workflows - Translation software can display both the key (resname) and original text (source)
• Automated translation support - Services like DeepL can translate the meaningful source text instead of keys
• Clearer context - Translators see "Home" instead of "navbar.home", making their work easier

Related Issues

• Closes #33041
• Related to [Translation] Allow specification of translation key in addition to content string in Twig templates #41797 (translation key specification in templates)