In other words, if you never fully learned to read in the first language that you were exposed to and start reading in a second language, what are the implications of this and how can you overcome them?
This was a very interesting question for me to consider, so I did quite a bit of research on the topic. The issue you describe is the plight of many uneducated or otherwise illiterate migrants or refugees, often children, who never received any formal schooling in their language.
An answer on this forum for teachers states:
The shortest path to literacy in English [or their second language] for such kids [that lack literacy in their first language], believe it or not, is helping them become good readers and writers in their first language. Those skills (print concepts, phonemic awareness, comprehension skills, etc.) are then transferred to their study of English [or their second language]. This kind of instruction is a great way to deliver needed content knowledge, too. Level of first-language literacy is a reliable predictor for how quickly and how well students will do when learning academic English [or any other second language].
As that excerpt states, having high levels of fluency in your first language can make acquiring a second language a much simpler process because you will have already been introduced to many grammar concepts and vocabulary terms.
An answer from this Quora question supports this assertion.
Language acquisition, per se, is not related to the ability to read and write. I have had many students from other countries come to my class (in the USA) with a strong verbal ability in their native language, but "illiterate" (at least functionally so), with respect to their ability to read and/or write in their own native language.
These students, however, given the motivation that they wished to communicate with their peers, have learned to speak English in a relatively short period of time. Their ability to read/write in the English language often ended up being better after a year in a US school than their ability to read/write in their native language after several years in their previous schools.
Speaking in the L2 at all time focusing on the commanding expressions could be a very good start while the commands are physically acted out at possible all time. People tend to learn first when they are exposed to acquisition rich listening environment. They should then be introduced writing and reading simultaneously.