How common is it for people to be literate only in a language other than their mother tongue? I know that ASL signing community members generally learn to be literate in English, since there is no widely accepted method of transcribing ASL, with all apologies to the inventors of Sutton Sign-Writing. But are there other examples in communities whose languages are spoken rather than signed?
As @jlovegren said, this is not an uncommon situation, especially in parts of the world that did not have a long history of literacy.
Take for instance Senegal as an example. The majority L1's in Senegal are Wolof and Puulaar, and a wide variety of other L1's are also spoken. Literacy in Puulaar is 10-30%, and is even lower in Wolof.
But the languages of instruction are French and Arabic (M Fall, 2011):
Wolof children in Senegal—West Africa—develop their first literacy skills in their second language (L2), French or Arabic, not in their first language (L1), Wolof. The Wolof language is primarily oral, and even though a written system has been recently developed, children still do not read and write in their L1
A similar situation exists with Bahasa Indonesia, the standardized official version of the Malay language in Indonesia. 23,000,000 speak it as an L1, but over 120,000,000 as an L2. Bahasa Indonesia is the primary language of instruction, and there is wide literacy in that language. So, there are many people who speak their native language orally but who can read and write Bahasa Indonesia.