Depends on the maid's willingness to learn. My maid finished secondary school (I think up to form 3). She picked up her english when she worked in Singapore (for 2 years), she can speaks and can read most instructions.
My Indo maid is very smart then. She said she finished high school and she speaks okay English. As far as I know, she had only worked in HK several years ago. Not sure if she can write very well in English, but she can read and speak English. She also read English books with my daughter. Of course, she got a little bit of accent, but still acceptable.