Beej said:
Learning another language or two or three will not confuse a child, in fact it will help develop their lexicon and linguistic skills faster than their unilingual counterparts. There is nothing to be lost by her learning Spanish, it will enhance her ability to speak English as well, and she will definitely never make a mistake about which words she's speaking in her family. She's a child, not an idiot...
Some people have more trouble with language than others, some pick it up fast. I've noticed people with an ear for music seem to pick it up quicker, at least the spoken part.
They also say people who learn an instrument, use other parts of the brain and make new connections that improve all around learning skills.
Languages always came hard for me, except Spanish which I learned young.
All of my kids are multi lingual, one has four languages.
The reality is the world is becoming a smaller place and English may not be the universal language it once was. Besides English being one of the more difficult languages to learn, Americans may just have to put out at least an equal effort to learn the language of there trading/business partners or be left behind in the world market.
Proper English is a myth (American isn't English), the grammar may be set and follow established rules, but both languages are dynamic and constantly evolving.
Picking up a second language may just be a part of the process. People tend to forget the object is communication and that tends to be bi or multi directional.
No matter how xenophobic your tendencies, in all likelihood you are eventually going to have to communicate with somebody that doesn't have your American language skills, whether you want to or not.