Upgrade/Install: Improve the appearance of language selection on install:

* Remove a stray chevron.
* Improve focus and hover styles.

Props garrett-eclipse, audrasjb.
Fixes #48927.
Built from https://develop.svn.wordpress.org/trunk@46887


git-svn-id: https://core.svn.wordpress.org/trunk@46687 1a063a9b-81f0-0310-95a4-ce76da25c4cd