I am an ESL teacher, I have worked with all grades, most cultures, all
levels of English acquisition.
A couple of things:
DO NOT tell a parent who is not a native English speaker to stop
speaking to their child in the home language. If the parent speaks
English, but not so well, this is worse for the childs English language
skills than simply speaking the home language at home. Why, you ask?
Simply it is because the parent is modeling bad speaking habits (this
has nothing to do with intelligence, it has to do with the parents level
of English ability), so the child comes to believe that this is the way
to speak English. I can usually tell which of my students has this
issue. They could be very competent readers, but speak like they have
only been in the country a few months. This impacts their ability to
spell well.
Even if the child has been in kindergarten, it does take quite a bit of
time to truly acquire English. There are words that this child may
never have heard at home that her classmates hear all the time. As to
your argument that the parent speak English, the PARENT may not know the
words or terms, either.
Children, as well as adults, acquire other languages in far different
ways and speeds. For instance, my son was abysmal in Spanish, three
years of Spanish I!, and now thrives in German. Why? I have no idea.
My daughter went straight to French II, skipping French I because we
moved to a new school district and gets straight A's. I have no idea
(sorry, can't take any credit, either!) why this happens, it is just how
they learn.
The kids I work with on a daily basis (in public school)are all learning
at different rates, despite the fact that I am teaching them all the
same thing! Some, at the end of first, are reading really, really
well. Others can't even identify all their letters. But they all
entered K as non-English speakers.
Depending on your students native language, vowel sounds may be very
difficult for her to distinguish between, and syntax may be different as
well, leading to confusion as to what sounds "right" to her.