I imagine you know this already: simply put, the younger a student is when acquiring a new language, the more accurate her/his pronunciation will be in the target language (ESL=English).
"Studies examining students' pronunciation after more than five years of exposure to the second language consistently find that the large majority of adults retain their accent when the second language is acquired after puberty, whereas children initiating second language acquisition before puberty have little or no foreign accent."
More on the subject here:
The Effect of Age on Acquisition of a Second Language for School
According to researchers, this has to do with brain "plasticity." This doesn't mean our brains slowly turn to a hard, dull plastic up to the point of senile dementia; on the contrary, it suggests that the human brain has the ability to change and develop (adapt) over time in relation to new information. The focus is on "neuronal circuits that respond to diverse stimuli and process information." In most cases, young learners have more plasticity:
"...in early second language acquisition (L2) beginning at between approximately age five or six, children often achieve native-like pronunciation and syntax within a relatively short period of a year or two. The ability of young children to achieve native-like proficiency in a foreign language in a rather short time is a reflection of a type of neural plasticity, and it appears to be related to the distinct characteristics of the young brain."
More here:
Neural Plasticity and the Issue of Mimicry Tasks in L2 Pronunciation Studies
If your student is a Korean native who acquired Russian as a child, she will most likely acquire English pronunciation in the same way a Russian adult would. On the other hand, if she hadn't been able to assimilate the Russian accent, perhaps then she will be coming to the lesson with pronunciation characteristic of a native Korean speaker.
Good Luck!
