My MC is an American fluent in Russian. Probably not native fluency but best described as near native, advance-level able to communicate easily in most situations.
The MC is in a meeting with a Russian businessman and his employee. The Russian knows the American speaks Russian, so he then switches to another language to speak to the employee because he doesn't want the American to understand. What language would the Russian switch to?
My original thought was Ukrainian or Belorussian, but I understand that those are close enough to Russian that a Russian speaker (in this case the American) could figure out what is being said. What other language possibilities are there for the Russians to speak? (Without it seeming contrived for the plot.) An unusual or "non-standard" dialect of Russian or even Ukrainian or Belorussian that would seem a natural fit.
It's fine if the American catches a familiar word or phrase, but he really doesn't understand what they are saying.
The Russian is well educated and traveled, fluent in multiple languages. His employee is less educated, likely some sort of technical school followed by time in the military.