The help menu for CKEditor doesn't tell authors that a language toolbar button is available if an administrator adds it to the menu bar. I would have preferred to have added it initially to the original Drupal 8 issue, but the very least that we can do is ensure that it is documented.
Currently from /admin/help/ckeditor we see this:
Accessibility features
The built in WYSIWYG editor (CKEditor) comes with a number of accessibility features. CKEditor comes with built in keyboard shortcuts, which can be beneficial for both power users and keyboard only users.
Generating accessible content
HTML tables can be created with both table headers as well as caption/summary elements. Alt text is required by default on images added through CKEditor (note that this can be overridden). Semantic HTML5 figure/figcaption are available to add captions to images.
I think we can change this paragraph:
<dt>Generating accessible content</dt>
<dd>HTML tables can be created with both table headers as well as caption/summary elements. Alt text is required by default on images added through CKEditor (note that this can be overridden). Semantic HTML5 figure/figcaption are available to add captions to images.</dd>To include something like this:
<dt>Generating accessible content</dt>
<dd>HTML tables can be created with both table headers as well as caption/summary elements. Alt text is required by default on images added through CKEditor (note that this can be overridden). Semantic HTML5 figure/figcaption are available to add captions to images. If your authors are multi-lingual you may want to enable the Language of Parts button from the <a href="/admin/config/content/formats">Text formats and editors</a> configuration.</dd>I'm a bit rusty, but just realized today that we hadn't included this.