How does the fact that pages in different languages ​​under the same URL affect SEO?

Question

Search Engine Optimization

I'm designing a website, I'm not a techie myself, please help, because I can't hire a SEO specialist at this stage.
So - there is a site https://uid.me , to which I have nothing to do, but this way I can explain it more easily.
There's a language selector in the lower right corner with a checkbox. You choose one of 8 languages, the page changes, but the URL remains the same.
In general, I want to order such a language selection system, my site will have 3 languages. A web developer doesn't care, maybe this way and that. So, I ask you - explain how this practice can affect SEO (emphasis on Google, if it matters), since I heard a lot that the robot does not like such things.
PS In this particular site https://uid.meit is clear that the page is immediately poured into the selected language, and through viewing the source code, it seems to be great to declare that the language is such and such.

Answer 1

To put it simply: it sucks, and it’s cached, don’t understand how

Answer 2

It is better to separate the pages with a prefix:
You can (and should) also send a Vary header:
But it doesn't really matter much, if at all.

Answer 3

I did this
Pay attention to
and META tags <link rel="alternate" hreflang="x-default" />
when changing languages.
Languages ​​- 2pcs (en and ru) and 3 pages:
/
/en/
/ru/
(I did everything listed according to the Google documentation)