Froxlor has Local File Inclusion via path traversal in API `def_language` parameter that leads to Remote Code Execution

Froxlor is open source server administration software. Prior to version 2.3.6, the Froxlor API endpoint `Customers.update` (and `Admins.update`) does not validate the `def_language` parameter against the list of available language files. An authenticated customer can set `def_language` to a path traversal payload (e.g., `../../../../../var/customers/webs/customer1/evil`), which is stored in the database. On subsequent requests, `Language::loadLanguage()` constructs a file path using this value and executes it via `require`, achieving arbitrary PHP code execution as the web server user. Version 2.3.6 fixes the issue.