Absolutely.

Can I notarize a document for a Spanish-speaking person, if Spanish is the language they will be speaking?

Absolutely.  If you speak and understand Spanish.

Can I notarize a document for a Spanish-speaking person, if Spanish is the language they will be speaking, but I don't know Spanish?

No!

At least not according to Colorado Notary Law.

I received a call today from a woman who wanted me to notarize a document for her mother.  No problem. Let's make an appointment.  Then she told me that her mother only speaks Spanish, but that she could interpret for her mother.

Sorry.  I told her that she would have to find a Spanish-speaking notary.  It doesn't matter if she is interpreting for her mother.  I must be able to communicate with her mother directly.

According to the Colorado Notary Law Primer:

"Under no circumstances should a notarization be performed if the Notary and the principal signer cannot communicate in the same language."

I do perform notarizations on documents in a foreign language.  Mostly German, because that is the language I know.  But I have notarized a document in Russian.  However, the person requesting the notarization for the Russian document was able to communicate in English.  That is the difference.

Hopefully this woman is able to find a Spanish-speaking notary.  Once I get through this Rosetta Stone course in Spanish, then I'll be able to serve her in the future.