When a website exists in several languages, a problem crops up that doesn't exist on a single-language site: Google has to decide which version to show each user based on their language and, sometimes, their country. Without any extra signal, it can end up showing the Spanish version to someone searching in English, or the other way around, simply because it has no way of knowing both versions exist and which one matches each search.
The hreflang tag solves exactly that: it's an annotation in each page's code that tells Google "this page has an equivalent version in this other language, at this URL". It's declared on every version, pointing to all the others, including itself (what's called self-referencing), and without that self-reference the whole implementation can fail to work properly.
A concrete example
A Spanish page (yourdomain.com/servicios) must declare that an English version exists at yourdomain.com/en/services, and that English page, in turn, must declare the same thing back, pointing to the Spanish version and to itself. If only one of the two pages declares the relationship, Google can ignore the signal entirely, because hreflang requires reciprocity between both pages.
The most common mistakes
Declaring hreflang towards a URL that actually returns a 404, forgetting each page's self-reference to itself, inconsistently mixing language code with country code (using en on one page and en-GB on another with no clear criteria), and the most frequent of all: implementing hreflang on the main listing pages but forgetting it on detail pages, like product listings or individual blog posts, leaving those pages without the signal the rest of the site has.
What if my website is only partially translated?
This is a common situation: maybe the blog is only in Spanish but the service pages exist in both languages. In that case, it's enough to declare hreflang only between the pages that do have a genuine equivalent version, without forcing a relationship with content that doesn't exist in the other language. Google handles perfectly well a site that has hreflang on part of its pages and not on the rest.
Frequently asked questions
Does hreflang improve my rankings on its own?
Not directly: its job isn't to improve ranking but to make sure Google shows the correct language version to each user. Using it correctly avoids losing visits by showing the wrong language, which is where the real impact lies.
Do I need hreflang if my site is only in one language but has several country domains (.es, .mx, .ar)?
Yes, even if the language is the same across all three: hreflang also handles country differences (for example, different prices or availability), not just language, so it remains relevant in that scenario.
How do I check if my hreflang is properly implemented?
Google Search Console has a dedicated hreflang report that flags reciprocity errors and broken URLs. There are also free external tools that analyse a URL and show exactly which hreflang relationships they detect and whether they're correct.