Firstly, I hope that you are arranging hosting that is located in Spain. Because that will be very important.
Google.es operates like most non-English localised Googles by offering two localisation options instead of Google.co.uk's one. The first option finds pages in Spanish from anywhere in the world while the second option finds pages from Spanish-based servers only.
In any case, the keywords will not be subject to translation: so Spanish keywords will tend to result in Spanish-language pages, with a smattering of other pages that happen to contain those keywords in Spanish, and English keywords will deliver the opposite. click here for an example.
Your client will benefit if you get your translator on board ASAP.