Where is the list with the languages?

I want to translate "German" to "Deutsch"

Yes now it makes more sense of that error and now I am also getting that error if not found AvailableLanguages.config fixed as you suggested

1.1 makes no difference
I think, because my site still doesn't have a setting like that

$settings = $site->getSettings('AvailableLanguages.config');

Update: changed line#20 to

if($settings && in_array($key, $settings)){

and now it's ok :)

Fixed in 1.1 but for me no crash :)

Got a crash right after choosing Configure -> Available Languages

[06-Apr-2022 18:34:36 Europe/Berlin] PHP WARNING: 2022-04-06 18:34:36 (CEST): "Undefined variable $checked" in file /var/www/ossn/components/AvailableLanguages/plugins/default/forms/avail_langs/settings.php (line 18)
[06-Apr-2022 18:34:36 Europe/Berlin] [#1649262876|FFO83970H47SIVGE8T4915L1I5] TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given in /var/www/ossn/components/AvailableLanguages/plugins/default/forms/avail_langs/settings.php:21
Stack trace:
0 /var/www/ossn/components/AvailableLanguages/plugins/default/forms/avail_langs/settings.php(21): in_array()
1 /var/www/ossn/libraries/ossn.lib.views.php(27): include('...')
2 /var/www/ossn/libraries/ossn.lib.views.php(50): ossn_include()
3 /var/www/ossn/libraries/ossn.lib.plugins.php(75): ossn_view()
4 /var/www/ossn/system/plugins/default/output/form.php(18): ossn_plugin_view()
5 /var/www/ossn/libraries/ossn.lib.views.php(27): include('...')
6 /var/www/ossn/libraries/ossn.lib.views.php(50): ossn_include()
7 /var/www/ossn/libraries/ossn.lib.plugins.php(75): ossn_view()
8 /var/www/ossn/libraries/ossn.lib.views.php(242): ossn_plugin_view()
9 /var/www/ossn/components/AvailableLanguages/plugins/default/settings/administrator/AvailableLanguages/settings.php(14): ossn_view_form()
10 /var/www/ossn/libraries/ossn.lib.views.php(27): include('...')
11 /var/www/ossn/libraries/ossn.lib.views.php(50): ossn_include()
12 /var/www/ossn/libraries/ossn.lib.plugins.php(75): ossn_view()
13 /var/www/ossn/libraries/ossn.lib.admin.php(218): ossn_plugin_view()
14 /var/www/ossn/libraries/ossn.lib.page.php(79): ossn_administrator_pagehandler()
15 /var/www/ossn/index.php(22): ossn_load_page()
16 {main}