I have installed Polylang plugin in my WordPress website. But once I activates the plugin then WordPress admin panel is showing HTTP 500 error. Frontend of the website is opening fine.
I have tried by increasing memory_limit to 256M in wp-config file but it has not solved the issue.
Remove plugin folder and upload again over FTP then test again just in case. If that doesn't help check PHP version and modules on your hosting. May be due outdated PHP or some module is missing that plugin require.