Support Underline & Language selection

[dd32]

As reported in https://wordpress.org/support/topic/issue-with-underline-rendering-in-forum/ & https://meta.trac.wordpress.org/ticket/7599 certain styles are not being respected properly.

This PR adds the needed tags for Underline and Language selection.

Note: For WordPress.org I'm likely to remove some of these once I figure out how :)

[dd32]

It turns out that the underline isn't coming from core, but rather from the calypso editor, which I guess BE is based on somewhere in the chain, it's actually wpcom/underline:

https://github.com/Automattic/wp-calypso/blob/f190f2f4b6f17d67c85a64061196f4cf9a949a4f/apps/wpcom-block-editor/src/default/features/rich-text.js#L15