Steps to reproduce the issue

Open TinyMCE plugin options in Joomla 4.
Save.

Expected result

Editor language should remain English.

Actual result

Editor language switches to Afrikaans. (first value of Language code dropdown)

Additional comments

The Language Code field in plugins/editors/tinymce/forms/setoptions.xml [line 106] is a filelist of the directory media/vendor/tinymce/langs/. The default value is en. However, there is no en.js file in that directory, so the default value is lost and replaced with af as soon as you tweak any TinyMCE plugin options.

Joomla 3 seems to have a similar issue, where you can't deselect a selected language code, but at least the default is empty, because of the different behavior of the dropdown. (the default value in setoptions.xml is still en)