Feature: #107875 - Improved DocHeader layout and unified language selector 

Unified language selector 

All backend modules with multi-language support now use a consistent language selector in the top-right area of the DocHeader. This includes:

• Content > Layout
• Content > List
• Content > Preview
• Record editing (FormEngine)

The language selector displays the currently selected language as the button text (for example "English", "German", and the special All languages) with a descriptive "Language:" prefix for screen readers. This provides immediate visual feedback about the active language while maintaining full accessibility.

The language selector now allows creating new page translations directly from the dropdown (except in Preview mode). The dropdown is organized with existing translations shown first, followed by a divider and a "Create new translation" section header. Languages that do not yet have a translation appear in this separate section.

Selecting a language from the "Create new translation" section will:

1. Create the page translation via the DataHandler.
2. Open the FormEngine to edit the newly created translation.
3. Provide a consistent workflow across all modules.

Unified module actions menu 

Modules with submodules or multiple "actions" now consistently display these options as a dropdown button in the DocHeader button bar. The dropdown button displays the currently active module or action as the button text, providing clear visual context to users.

For accessibility, each dropdown includes a descriptive label:

• "Display mode" for the Content > Layout module view selector (layout vs language comparison)
• "Module actions" for general module or submodule navigation
• Custom labels defined by individual menu configurations

These labels are implemented using visually hidden label elements, ensuring screen readers announce both the dropdown's purpose (for example, "Display mode:") and the current selection (for example, "Layout").

The dropdown is automatically hidden when only a single action is available, reducing visual clutter and showing the dropdown only when navigation choices are actually available.

Examples include Administration > Users with Overview, Users, Groups actions, as well as Content > Status with "Pagetree Overview", "Localization Overview", and more.

This replaces the previous inconsistent mixture of:

• Back buttons
• Inline button groups
• Module menus in different locations

Reorganized DocHeader layout 

The DocHeader now has a more logical structure:

Top row (navigation bar):

• Left side: Breadcrumb navigation showing the current location
• Right side: Language selector (when applicable)

Second row (button bar):

• Left side, group 0: Module actions dropdown (when applicable)
• Left side, groups 1+: Additional action buttons and context-specific buttons (Save, Close, and so on)
• Right side: Functional buttons such as "Reload", "Bookmark", or "Clear cache"

Module actions dropdown 

Controllers can use the existing ModuleTemplate::makeDocHeaderModuleMenu() method to automatically create a module actions dropdown based on the module configuration:

$view = $this->moduleTemplateFactory->create($request);
$view->makeDocHeaderModuleMenu();

For backward compatibility, the MenuRegistry API is automatically converted to module actions dropdowns.

Language selector integration 

The language selector is integrated via the DocHeaderComponent::setLanguageSelector() method.

First, inject the \TYPO3\CMS\Backend\Template\Components\ComponentFactory into your controller:

use TYPO3\CMS\Backend\Template\Components\ComponentFactory;

public function __construct(
    private readonly ComponentFactory $componentFactory,
) {}

Then build the language selector dropdown with language items:

// Get available site languages for the current page
$siteLanguages = $this->site->getAvailableLanguages($pageRecord, $backendUser);
$currentLanguageId = (int)$moduleData->get('language');

// Build dropdown button
$languageDropdown = $this->componentFactory->createDropDownButton()
    ->setLabel($languageService->sL('core.core:labels.language'))
    ->setShowLabelText(true)
    ->setShowActiveLabelText(true);

// Create dropdown items for each language
foreach ($siteLanguages as $siteLanguage) {
    $languageId = $siteLanguage->getLanguageId();
    $isActive = $currentLanguageId === $languageId;

    // Build URL for language selection
    $href = $this->uriBuilder->buildUriFromRoute('web_layout', [
        'id' => $pageId,
        'language' => $languageId,
    ]);

    $item = $this->componentFactory->createDropDownRadio()
        ->setHref($href)
        ->setLabel($siteLanguage->getTitle())
        ->setIcon(
            $this->iconFactory->getIcon($siteLanguage->getFlagIdentifier())
        )
        ->setActive($isActive);

    $languageDropdown->addItem($item);
}

$view->getDocHeaderComponent()->setLanguageSelector($languageDropdown);

The language selector component is automatically rendered in the top-right area of the DocHeader navigation bar.

$dropdown->setLabel('Display mode')  // Descriptive label for context
    ->setShowLabelText(true)         // Show label text
    ->setShowActiveLabelText(true);  // Show active item label