Content Translation shows a confusing set of warnings even when config is syncing

Problem/Motivation

If you install Content Translation, you get a pair of warnings if there's only one language.

This is a very weird thing to see during config sync, during which it can be presumed that you have only one language (plus Content Translation) for a reason. Indeed, content_translation_install() already returns early during site installation, probably for this reason exactly.

The fact that it doesn't also return early during config sync is likely an oversight.

Steps to reproduce

Apply a recipe that installs Content Translation on a monolingual site. Or, heck, sync config in such a way that Content Translation gets installed during the sync.

Proposed resolution

If $is_syncing is true, content_translation_install() should return early.