Currently it is impossible to add/remove sitelinks in Russian UI. I would appreciate if someone will look into that as soon as possible

Now I tested to use Russian interface on Wikidata in this page (https://www.wikidata.org/wiki/Q24038596?uselang=ru), and it now works normally. I too had the problems, but I not using Russian interface on Wikidata - I am using English interface.

started to experience errors with JavaScript

What errors?

I'm afraid without more details It is unlikely that we will be able to determine why it was happening.

In T202346#4527606, @Marshmallych wrote:

I too had the problems, but I not using Russian interface on Wikidata - I am using English interface.

It's possible that another ticket was filed for this issue?

In T202346#4538207, @Addshore wrote:

What errors?

I'm afraid without more details It is unlikely that we will be able to determine why it was happening.

Basically, subheaders had new localisation in edit buttons (‘править’), and all AJAX for them didn’t work at all. Properties had old localisation (‘редактировать’) and AJAX worked for them.

Did a purge of the page fix the issue or did it persist?

all AJAX for them didn’t work at all

As in the AJAX failed, or it wasnt there and you always went to the special pages?

Sounds like something died early on in the JS, but without knowing what was in the JS console this could be pretty hard to identify or link to anything else.

Purge did nothing. No errors were in console as far as I am aware. Any action was being sent to the fallback, yes, so events were probably not subscribing to links with new localisation.