From what I've heard from others (I don't speak Spanish), many of the recent Hispanic immigrants to the U.S. are fluent in neither English nor Spanish. I'd guess their Spanish is the street lingo you mentioned.
I visited Mexico City many years ago. Even then the poverty was incredible. Miles and miles of slums. I imagine it's much worse today.
There was a case here where a judge got in big trouble because he ordered a Mexican national to learn English or she wouldn’t get her child back; the court had tried every available translator they could locate and not one spoke her dialect.
The judge barely escaped with his skin, the kid was returned to his certain poor future and the woman was never able to comply with the coourt ruling.
She just sort of dropped off the radar.
Nashville, Tn has seen the numbers of illegal immigrant newcomers more than double each decade for the last 25 years.
I was a volunteer reading tutor at a local Nevada middle school that was trying to bring kids up to grade level. Over 90% were Hispanics, the rest were whites with attitudes.
The teachers told me of two problems: One was that many of the Hispanics couldn't read in any language. The other was that while the Hispanic kids wanted to learn English, their fathers forbade them from speaking or reading English at home. "Spanish is your heritage." they were told. These parents, legal or not, are colonists, not immigrants.