While I agree there could be an issue with some RHD vehicles... I would never try to solely rely on hand signals. The more communication the better... As long as it is only between a single spotter and the driver.
It is absolutely the spotters responsibility to realize when they are spotting with a vehicle that is LHD or RHD and then they should adjust their language accordingly. If the Driver is on the opposite side that you are accustomed to, then as a spotter, you will need to adjust how you speak with them.
I am by no means saying you shouldn't use hand signals... You should always use both in my opinion!
In every case. The drivers, those who will be spotters, and those who will be spectators on a trip. Should always sit down and talk about exactly what language they are using. What role everyone is playing, what signals, what language, and what you feel are best practices. Safety is always the most important thing on the trail! And everyone being on the same page helps to keep safety in everyone's thoughts!
For multi day trips, this should be a daily occurrence.