What happened?

1. Install the German language pack and set the administrator language to German (de-DE).
2. Open a site module of type Menu for editing (Cassiopeia is the default site template).
3. Open the Advanced tab and look at the Layout field.

Version

6.2

Expected result

The Cassiopeia layout collapse-metismenu is shown as "Dropdown einklappbar"
(TPL_CASSIOPEIA_MOD_MENU_LAYOUT_COLLAPSE-METISMENU in language/de-DE/tpl_cassiopeia.sys.ini).

Actual result

It is shown as "Collapsible Dropdown", the en-GB string. The same happens for every
layout name that a template translates in its .sys.ini.

System Information

Joomla 6.2.0, PHP 8.4. The code involved is unchanged in 6.2-dev, 6.3-dev and
7.0-dev; 5.4-dev has the same logic (without the language fallback chain added in 6.x).

Additional Comments

Two calls load tpl_<template>.sys.ini in the same request:

1. The position field translates the position names via
   ModulesHelper::getTranslatedModulePosition()
   (administrator/components/com_modules/src/Helper/ModulesHelper.php). It loads
   the file in the current language with $default = false, so the en-GB file is
   not loaded:

   $lang->load('tpl_' . $template . '.sys', $path, null, false, false)

2. Later, ModulelayoutField (libraries/src/Form/Field/ModulelayoutField.php)
   loads the same file with $default = true:

   $lang->load('tpl_' . $template->element . '.sys', $client->path)

   Language::load() first loads the en-GB file, which has not been loaded yet;
   array_replace() in loadLanguage() overwrites the German strings. The de-DE
   file is then skipped because it is already in $this->paths, so the English
   strings remain.

Minimal reproduction without the backend (run from the Joomla root):

<?php
define('_JEXEC', 1);
define('JPATH_BASE', __DIR__ . '/administrator');
require JPATH_BASE . '/includes/defines.php';
require JPATH_BASE . '/includes/framework.php';

use Joomla\CMS\Language\Language;

$key  = 'TPL_CASSIOPEIA_MOD_MENU_LAYOUT_COLLAPSE-METISMENU';
$lang = new Language('de-DE');

$lang->load('tpl_cassiopeia.sys', JPATH_SITE, null, false, false); // ModulesHelper
echo $lang->_($key), "\n";                                          // Dropdown einklappbar

$lang->load('tpl_cassiopeia.sys', JPATH_SITE);                      // ModulelayoutField
echo $lang->_($key), "\n";                                          // Collapsible Dropdown

Possible fixes: load the file with $default = true in ModulesHelper (as the
other callers do), or make Language::load() re-apply the requested language
after it has loaded the default language for an already loaded file.