Panel Translation error

[beebmx]

Describe the bug
There is a problem in the panel with the translation (not all languages).
If you set (por example) spanish it throw an error
Undefined array key "es"

To Reproduce
Steps to reproduce the behavior:

1. Download a plainkit project.
2. Create config/config.php
3. Enable languages with 'languages' => true
4. Create a language file site/languages/es.php

<?php

return [
    'default' => true,
    'code' => 'es',
    'direction' => 'ltr',
    'locale' => 'es_MX',
    'name' => 'Español',
    'url' => '/',
    'translations' => [
        'lang' => 'Español',
    ]
];

5. Go to /panel
6. An empty page will show

Note: If the translations key is empty the no error will show.

Expected behavior
The panel page (installation) should be shown

Screenshots

Kirby Version
The version affected is 3.5.2 in the 3.5.1 and below worked as expected.

Console output

Desktop (please complete the following information):

• OS: macOS 11.2.1
• Browser: Chrome, Safari
• PHP: 7.3, 7.4, 8.0

Additional context
There is no problem with the templates, this issue only affect to the panel.

[afbora]

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