hreflang
A declaration that two pages are the same content in different languages, so a search engine shows each reader the right one.
The links have to be reciprocal: if the English page points to the Serbian one, the Serbian one has to point back, and both to themselves. A one-way declaration is ignored, which is why half-translated sites see no benefit from it at all.
A page whose translation does not exist yet should have no pair. Pointing at a page that is empty, or that quietly serves the other language as a fallback, is worse than pointing at nothing.