For IP users, the interface language variant is changed automatically when changing the content variant. However, for logged-in users, that isn't the case and these two are not in sync.

In theory, logged-in users are expected to select their user interface language in Special:Preferences. However, this fixed option can lead to confusion, as the interface language can differ from the actual content.

Serbo-Croatian example
For example, if we go to the sh.wiki main page and select sh-cyrl, the content is transliterated to Cyrillic, but the interface remains in Latin, which is not expected by users/readers.

Serbian example
The opposite case is Serbian Wikipedia. If we visit the sr.wiki main page and select sr-el, the content is transliterated to Latin, but the interface remains in Cyrillic, which is not expected by users/readers.

Proposed solution
Perhaps there should be an option to make interface language follow the default language conversion variant, or vice versa?