What should a Pilot do if they encounter foreign military jets?
According to ICAO conventions the offical international language of aviation is English therefore it is unlikely that Russian fighter pilots will speak in Russian to a foreign pilot. If the pilot overlfying American waters, it's unlikely that Russian fighters are free to intercept planes in American airspace.
If such scenario happens, the pilot can report the situation to the ATC, then the situation will quickly escalate. If the private pilot is in American airspace and Russian fighters are not authorised, the private pilot should be safe following ATC directions and/or continue the route as planned. USAF will take care of the rest.