Partially multilingual site in D8

Is it possible to create a partially-multilingual site in Drupal 8, without having a bunch of nonsense URLs? Details...

Site setup:
- Most of my content is in the primary site language (English, in this case).
- Some content items have been translated into a second language (Spanish, in this case).
- The rest of the site hasn't been translated into the second language, and there are no plans to do so.
- The pages on the site have URL aliases.

Now, let's say I start in English, and go to node/32, aliased as /my-beautiful-dog, which has been translated into Spanish. I have a language switcher block enabled, so I click on Spanish, and get to page /es/mi-hermoso-perro. So far, so good.

Now I use the site's main navigation to go to the site About page, which is only available in English, at node/3 aliased to /about. However, since I am viewing the site in Spanish, the URL I get to is instead es/node/3 (no alias). Which shows the page in English anyway (since that is all that is available).

So... The problems with this are:
- The URL I am on is ugly (es/node/3)
- Two pages on my site exist with the same exact content (the About page in English), with different URLs. This is terrible for search engines.
- Really the URL of es/node/3 should never be allowed to exist. This page is only in English, so when I visit node 3 I should never be in Spanish.

In a Drupal 7 similar situation, I wrote a quick module of a few lines, I think using hook_init() or hook_boot() or something like that, which noticed I was on an English-only page and did a drupal_goto() that redirected back to the English URL. It worked, though it was a bit brute force...

I'm not sure how to do that in Drupal 8... or even if that is a good idea. Any ideas of how to resolve this? Thanks!