Feature: #89334 - Add generic TranslatorInterface 

Description 

A new \TYPO3\CMS\Core\Localization\TranslatorInterface has been introduced to provide a clean abstraction for translating labels in TYPO3.

The interface defines two methods:

• translate() translates a label using its identifier and domain, supporting argument interpolation, a default fallback value, and per-call locale overrides.
• label() resolves full TYPO3 label reference strings, for example LLL:EXT:core/Resources/Private/Language/locallang.xlf:my.key, and delegates to translate(). This method serves as the interface-based equivalent of LanguageService::sL(). The key differences are that it returns null when a label cannot be resolved instead of an empty string, and supports argument interpolation, default values, and locale overrides.

LanguageService now implements this interface, making it possible to type-hint against the interface instead of the concrete class.

Example usage:

use TYPO3\CMS\Core\Localization\TranslatorInterface;

final class MyController
{
    public function __construct(
        private readonly TranslatorInterface $translator,
    ) {}

    public function someAction(): void
    {
        // Translate by identifier and domain
        $label = $this->translator->translate(
            'button.save',
            'my_extension.messages',
        );

        // Translate by identifier and filename (discouraged)
        $label = $this->translator->translate(
            'button.save',
            'EXT:my_extension/Resources/Private/Language/locallang.xlf',
        );

        // Translate with arguments
        $label = $this->translator->translate(
            'record.count',
            'my_extension.messages',
            [5],
        );

        // Translate with a default fallback
        $label = $this->translator->translate(
            'missing.key',
            'my_extension.messages',
            [],
            'Fallback text',
        );

        // Label reference with arguments and default
        $label = $this->translator->label(
            'my_extension.messages:record.count',
            [5],
            'Fallback text',
        );

        // Resolve a full label reference string with file and LLL prefix (discouraged)
        $label = $this->translator->label(
            'LLL:EXT:my_extension/Resources/Private/Language/locallang.xlf:button.save',
        );
    }
}

The label() method accepts the following reference formats:

• my_extension.messages:my.key
• EXT:my_extension/Resources/Private/Language/locallang.xlf:my.key
• LLL:EXT:my_extension/Resources/Private/Language/locallang.xlf:my.key

The LLL: prefix is optional and removed before resolution.

Impact 

Extension developers can now type-hint against TranslatorInterface instead of the concrete LanguageService class. This improves testability and decouples code from implementation.

The translate() method of LanguageService has been extended with two additional optional parameters:

• $default a fallback value returned when the label is not found
• $locale allows overriding the locale on a per-call basis