I’m not certain this is as big an effect as you might imagine.
I’m an immigrant in Portugal, I have been here five years - and I am far more literate in Portuguese than a good many of the people I encounter here, who are born and bred Portuguese. Sure, they speak better than me, of course, and it took me a while to realise that many of them could barely read or write - but the educational system in rural Portugal did not, and seemingly still does not, produce people with anything above bare-bones literacy.
This isn’t a judgment - purely an observation that literacy is something that translates across language barriers for the literate quite readily, and poor education results in poor literacy - not being foreign.
I’m an immigrant in Portugal, I have been here five years - and I am far more literate in Portuguese than a good many of the people I encounter here, who are born and bred Portuguese. Sure, they speak better than me, of course, and it took me a while to realise that many of them could barely read or write - but the educational system in rural Portugal did not, and seemingly still does not, produce people with anything above bare-bones literacy.
This isn’t a judgment - purely an observation that literacy is something that translates across language barriers for the literate quite readily, and poor education results in poor literacy - not being foreign.