multi lingual windows

I am wondering, is the best way to make my PC bi-lingual to have two partitions, one is Spanish and one in English? or can I just have windows change itself into Spanish mode somehow with language compatibility?

You can have the two modes act as users. The english mode a user, and the spanish mode as a user. To change the language go to Control Settings > Regional and Language Options. Or you can just change the language there when you want.