How do hreflang errors affect rankings on multilingual sites?

Short answer: hreflang is not a ranking boost by itself; its job is to match the right language and country version to the right user. When it is broken, Google serves the wrong version: the wrong page competes in the wrong market, clicks and conversions drop, and versions cannibalize each other.

Most common errors and their effects

• Missing reciprocity: if page A references B but B does not reference A back, the pair is ignored.
• Invalid codes: use ISO 639-1 for language and ISO 3166-1 for country; en-uk is invalid, the correct form is en-gb.
• Missing x-default: it stays undefined which version users without a match should see.
• Canonical conflict: if hreflang points to one URL and the canonical to another, the signals contradict each other.
• Wrong targets: hreflang must point only to indexable URLs returning status 200; redirects and noindex targets break the chain.

For diagnosis, read Search Console data, crawler reports, and log files together. Measure the fix per market against an impressions and clicks baseline; the first measurable result is the correct version appearing in the correct country.