Add support for language of parts to TinyMCE

[StephDriver]

We are not currently compliant with WCAG 3.1.2 Language of Parts

The human language of each passage or phrase in the content can be programmatically determined except for proper names, technical terms, words of indeterminate language, and words or phrases that have become part of the vernacular of the immediately surrounding text.

This is due to user-content, but we are not facilitating non-technical users as to how they mark up phrases in other languages.

We do currently set hte language of the page (as per WCAG 3.1.1) but where words or phrases are not in that language, they need to be marked up as per technique H58.. We do not need to do this with words in common usage, such as in situ but for clarity, we should consider maintaining a list of words not being marked up.

The following examples from the #4194 should be considered when implementing this feature.

• Sample 2, items 39 and 54
• Sample 3, items 20 and 35