Windows 7 - Activate English version with Spanish version key?

Hi, newbie and non-techie, please be gentle! Posted in Tutorial section about changing key code, but not sure if it's the right place so will copy here too.
I am buying a new PC in Spain but want Windows 7 Home Premium in English, which does not seem to be available anywhere here. So my techie guy is downloading an English version (pirate???) and at same time buying a genuine Spanish version. He says he will then use the key code from the Spanish pack to validate the English version.
Is this possible, and could I encounter any problems later? How can I be 100% sure (before I leave the shop!) that I have a validated, genuine English version of Windows 7 that will continue to work indefinitely?

I have read that the keys are language-version-specific. Others may know for sure.

Here is how to doublecheck validation before leaving the shop. Ask for the Spanish pack key insert. Run Belarc Advisor.exe and scroll down to see the installed keys on machine: compare audit's Windows 7 key to the key your shop provided given on package insert.

Now go to Start>right click Computer>Properties and scroll down to confirm Windows is activated.

Be sure to keep the Spanish package key insert with the paperwork from your shop. Ask for a DVD copy of the Windows 7 installer to use for reinstalls or repairs.

You can be 100% sure that everything will be OK by reading the EULA and purchasing the legit copy of Windows 7 yourself. You can purchase & download form the MS website;
Windows 7 Home Premium Upgrade - Buy and download from Microsoft Store

Thanks for this, Greg. I will run Belarc before taking it home.
I have seen the machine running (in English) this morning (still in the shop because I'm waiting for the monitor to arrive - everything arrives here mañana!) and the System screen shows Windows 7 as activated, with the product ID which corresponds to the number on the sticker, which he's already put on the tower case - he has the OEM CD and package for me, but unfortunately I didn't see the sticker on the package first. Is there any possibility it could have come from anywhere else, and not be legit? I've read elsewhere that activation keys are readily available from dubious sources. What about the product key stickers?
I know it must sound as though I'm paranoid, and that I don't trust my supplier - it's more a case of not trusting myself to have understood everything he tells me. My Spanish is good, but when it comes to technical talk it can let me down, hence I need my OS in English - although even in English I get confused easily!
rsvr85 - I appreciate your comment, and in retrospect, I wish I had stumped up the extra for Ultimate and not had to worry about this - but I had already committed myself by the time I posted here.

Thanks again guys for the suggestions. I'm now a happy Windows 7 user - all activated, validated etc.etc., and yes, I was worrying about nothing. No problems at all, and I'm enjoying finding my way around the new system - in my own language. The Spanish keyboard is a bit irritating, but no difficulty getting used to a Spanish mouse whatsoever. Happy days!