What language do Americans speak as a 2nd language?

If Americans speak a second language, it is most likely to be Spanish.

However, although most Americans study foreign languages in school, they normally do not speak any language other than English either at home or at work/school. Only immigrants are likely to do this.

Americans do speak regional versions of English, but these usually do not vary enough from Standard English to be called "dialects".  Regional English in the US is mostly a matter of accent and some vocabulary or expressions.

I am fluent in Army, and passable in Navy & Air Force ..............

imo most americans don't have a talent for learning languages lol

to answer your question, they should technically speak their parents' first language 

Broken local slang. 

Spanish or french

All depends on the individual: Most who know a second language at least proficiently 

learned it at home, as few who only took it up at school actually bother to learn much.

I personally speak either French or Spanish with most of my family and close friends.

At work though, I mostly use English with coworkers as it's what they speak or prefer.

There is no second language in the US, with the wide diversity over here, that clearly depends on the individuals. Although English is the most common language it's not even the official language here, there is no official language by law. Spanish is however very easily the second most common language here.

Depends on where you live?  If you live in Canada you should know English and French.  If you live in USA you need to speak Spanish and English.  You can go anywhere in the world with those two languages and be understood.  A professor once said, "if you teach your children one thing it would be another language to help him get a job and keep it."  From a translator to working in with the public.  They always need someone who can do that. 

Only english instead they are people of immigrant backgrounds I would believe. 

The USA is a FAR larger country than the Philippines, with a far wider range of ethnicities within it. Spanish is turning into a second language for many US citizens, but obviously more so in the south and west.

There is no specific 'second language' in the USA, just as there isn't one in the UK or France of Germany or ... but it's normal for many of their citizens to learn a foreign language at school, if only to a lowish level.