Bug when inline-edit post with single language

Hi,

Thank you very much for your plugin. You’ve done a lot of great work!

I found a small problem in the plugin. It is located in the file: /js/post.js on lines 29-30:

filter_terms( lang ); // initial filter for category checklist
filter_pages( lang ); // initial filter for parent dropdown

When I have only one language added in Polylang, the value of lang is undefined and this hides all elements such as categories in inline-edit mode.

This is because when I have added one language in Polylang, the column with languages is not visible on the list of posts. When it is not visible, the lang value cannot be obtained in JS.

Screenshots:

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