Interesting bits of an interview with the series author. So, instead of Russian, it was meant to be that the female main character was isekai'ed. From the article:

In this version, the reincarnated heroine would speak Japanese to the protagonist, feeling confident that he couldn’t understand her.

The twist would be that the protagonist was also a reincarnated person who could understand Japanese perfectly. This humorous misunderstanding was meant to form the core of a short story.