Were Nikita's messages actually in response to what Dennis was saying, or was this the equivalent of him plugging his ears and refusing to communicate with his brother just because the messages were in Russian?
They are in response to what he's saying. Denis appears to know English since he might be a Spanish (?) resident.
"That's right, only the grave will fix you".
"Yep, you just exude humanity to the point where people are repulsed by you or even want to kick your ass. You don't have a friend or a like-minded person...you're worse than a monkey, they are social species at least, while you are a lonely angry sociopath".