In my experience monolingual people underestimate the effort it takes to use your non-dominant language(s). For example, I could theoretically communicate in Sp

In my experience monolingual people underestimate the effort it takes to use your non-dominant language(s). For example, I could theoretically communicate in Spanish, but the effort it takes to deal with romance language conjugation is a legitimately unpleasant experience for me. Mochizuki might know English, but it might be the case that using it is an unpleasant experience to him. In addition, just as there are English proficiency requirements for international students in the US, I feel it would be unreasonable to pursue higher education in Japan without even knowing enough Japanese to talk about math.