The most expensive misunderstanding in Spanish language search is treating it as an English programme with a translation step bolted on the end. The pages come back grammatically perfect and commercially useless, because the words people type are not the words a translator chooses. Anybody buying SEO here should test for this in the first meeting, since almost every agency claims the capability and a minority actually have it.
Spanish language SEO is not English work run through a translator
Search vocabulary is colloquial, regional and frequently ungrammatical. Buyers type the informal name of a product, the brand used as a generic noun, an abbreviation, or a phrase with an accent dropped because they are typing quickly. A translation workflow produces the formal register and misses all of it.
The test is simple. Ask a prospective supplier to name two phrases in your category where demand in Spanish diverges from what a direct translation of the English would produce, and to say which one they would build a page around. A team that does its own research in the language will answer in a few minutes. A team that outsources translation cannot answer at all, because the question has never come up in their process.
Decide which Spanish speaking markets the Madrid site is actually for
This is the structural decision that everything else depends on, and it is usually made by accident. A site written here can serve domestic buyers only, or it can chase demand across the wider Spanish speaking world where the same language hides different vocabulary, different currencies and very different competition.
Both are legitimate. What fails is pretending to do the second while building for the first, which produces pages that rank in no market particularly well. If you are going wider, the site needs a deliberate structure with language and regional signals set correctly, and somebody has to own the fact that a term for a product in one country can mean something else entirely in another. Ask each supplier to state a recommended structure, a reason, and the cost of reversing it later.
An SEO agency that measures in English will mismeasure your demand
Tools default to certain markets and certain assumptions, and reports assembled from those defaults quietly understate domestic demand and overstate the competitiveness of phrases that matter. Watch for a proposal where every example phrase is a clean, formal noun. Real Spanish SEO data is messier than that, and a supplier that has actually looked will show you the mess.
There is a second measurement trap here, which is seasonality. Large parts of commercial activity in this country effectively pause for several weeks in late summer and again around the winter holidays. A quarterly SEO report that spans one of those periods will show a collapse that has nothing to do with the work, and an unscrupulous report will show the recovery afterwards as a triumph. Insist on comparisons against the same period a year earlier.
What the proposal has to reveal about the writing line
Ask for the content production to be costed separately from strategy and from technical work, and ask specifically who writes: an employee, a freelancer, a translation vendor, or a generative tool with a human pass. There is no single correct answer, but the price should reflect it and you should know. The most common unpleasant discovery in this category is that the attractive monthly figure covers coordination only, and every published page is billed on top.
Ask too whether the writer has ever worked in your sector, and ask to read something they produced. Subject knowledge shows up within a paragraph and cannot be faked by a briefing document.
Rights to the copy, the glossary and the accounts
Analytics and console properties, the tag container, the content management system and the domain registration belong in accounts held by your company with access granted to the agency. Articles and page copy transfer to you on payment, with editable source files rather than exported documents.
One asset specific to multilingual work deserves naming in the contract: the terminology glossary. It records how your buyers refer to each product, in each market, with the variants that earn traffic and the ones that do not. It is built slowly out of real data and it is the most valuable thing a good SEO partner produces that nobody thinks to ask for. If it lives only in the agency's project tool, you lose a year of learning on the day the relationship ends.
A commercial measure that survives a quiet August
Pick one number expressed the way your business talks: qualified enquiries, bookings, or orders, with the cost of each beside it. Keep visibility and position data in the report as diagnostics rather than headlines. Then agree explicitly how holiday periods are handled in the comparison, because the alternative is a review meeting spent arguing about a graph that only reflects the calendar.
Agree the timeline too. Technical remediation lands quickly. Content and citations take a quarter or more to move anything, and a serious SEO programme looks flat before it compounds. Write down in advance what early evidence would justify continuing, so the first review is about facts.
If you would rather send one requirement to several firms and compare like with like, set out the brief once and ask for answers in a fixed shape. For the wider view of how these suppliers are structured, start from the overview of SEO agencies. If the constraint turns out to be the platform rather than the pages, talk to local software teams or product firms working on applications, and if you are comparing supplier markets, look at agencies on the coast or firms in the neighbouring country.