Uncaught type error for tinyMCE.addI18n function

[SteelWagstaff]

When users load the visual editor, they see an Uncaught TypeError: tinyMCE.addI18n(...) is not a function message in the browser console. I believe our usage comes from

pressbooks/languages/tinymce.php

Line 3 in 9658b99

$strings = 'tinyMCE.addI18n( "' . _WP_Editors::$mce_locale . '.strings", {

. Not sure what may have changed with recent WP releases to break this.