A vital element of a sound notarization is the ability for the Notary to communicate directly with a signer to verify identity, willingness and awareness, or to recite an oath or affirmation, among other things. If direct communication with the signer is not possible due to a language barrier or some other circumstance, the notarization must be refused.
You can, however, refer the signer to a Notary who speaks the appropriate language. Sometimes foreign consulates, which normally have officers with notarial powers on staff, may be a convenient solution for the foreign language-speaking signer.
Additionally, Notaries should never rely on an intermediary or interpreter in performing a notarial act. Besides the possibility of misunderstanding, a dishonest interpreter may have a motive for misrepresenting the facts.