This is a result of the multilingual software we use on Diff, Polylang. At least in part. It creates a language variant for each taxonomy (categories/tags) when used. This way each category or tag can be written and displayed in the appropriate language (so you don't see a post written in Russian with the English category "Campaigns"). Ideally when editing a post you'd see something like "Campaigns (RU)" or "кампании" in the selection list.

The editing interface doesn't indicate which tag is which language. This can be done manually, but with 216 categories and 3,513 tags is a lot of work.

I have a task to try and limit the number of categories contributors can see/use to the primary six we have in the site navigation. This would make the scope of "clean up" much smaller and more manageable to translate.