If it's Ultimate you can use the Spanish language pack via optional Updates.
Otherwise what I would do is find a Spanish installer for that version to clean reinstall. There are reports that keys are language-specific, but other real-world tests report that same key will work for a different language within a certain region. If not you'll need to provide a Spanish key.
If you bought Builder's OEM then call MS Customer Service to tell them you installed it on the wrong machine, which is the kind of legit excuse they are repeatedly reported to accept to allow moving retail OEM. In your case they may want you to wait until you move it, or will release the key for reuse if that's possible.