The plugin does not work in multisite or if WordPress is installed in a subdir. The fix however is simple.
On line 67 change this:
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>?page=oneBackendLanguage">
to this
<form method="post" action="?page=oneBackendLanguage">
This plugin doesn’t work with Multisite.
You can navigate to the options in one of the subblogs but submitting redirects back to the main blog and the settings aren’t saved for the subblog.
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