From eurekalert.org: Mental math dependant on language:

The language most bilingual people use to mentally solve math problems isn't necessarily their native language or even the language that is most prevalent in their environment. Psychological research shows it's the language in which they were first taught math - a finding with educational implications, especially for areas with high concentrations of bilingual persons. Texas A&M University psychologist Jyotsna Vaid's research has found that bilinguals' preferred language for different mental arithmetic activities is the language in which the associated skills have been acquired.