I notice that both Office 2011 and Office 2011 Spanish edition are available at Amazon here in the US in three-user home and student editions. However, the Spanish version costs only two-thirds of what the English version does. Can the menus of the Spanish version be displayed in English? Are there any differences in functionality between the two products?
Some MS programs will flat-out tell you that they were made for Spanish only, like IE and won't even install.
Regardless, I had a USA English PC with Spanish Office XP (meaning version 2002). My regional settings are for English / USA, in case you were wondering --I know what multilingual programs on Windows act like. Office 2002 only came with the Spanish strings and has no English workarounds at all. Unless MS is more generous now in spite of the bad economy, assume nothing improved for Office 2011, especially if both sell in your local country.
For more evidence that you won't often get multilingual-bundled installers from Microsoft, you can confirm MS downloads like Internet Explorer are split by language. Functionality is the same --the only difference I did notice is that Spanish 2002 has English/USA spelling dictionaries built-in while English 2002 didn't have all of the Spanish dictionaries (Spanish grammar and thesaurus were missing I think.)