http://texasnotaryblog.com/2014/texas-notary/what-not-to-do-with-foreign-language-notarizations/
Texas law does not require you to understand the language that a document you are notarizing is in. This being said, it opens you up to more liability if you are unaware of what the document is actually saying.
My understanding after reading this is: a notary must (1) make sure signer is who he/she claim to be; (2) make sure signer understsand what the document is.
In practice some notary might refuse to do this, esp. when they never dealt with foreign language document before. But in a metropolitan where many immigrants live, chances are some notary would understand the true nature of the law regarding foreign language documents.