[Jakarta ex-pat]

"Learn to read and write Spanish and retain your advantage over the illegals."

Yes, reading and writing in your mother toungue is critical.

If you can't read and write in your mother tongue, you will not learn a second language fluently.

[RightWhale]

Have you come across a street urchin selling peanuts to tourists in more languages than one needs for a doctorate in philology? Sure, speaking is one thing and writing another.