Beautiful chairs!
It's worth checking if you can clean them first with natural soap. Over the years, wood will get dirty (contact with the human skin). A natural soft soap (and not soaking wet!!) will clean off a lot of the dirt and reveal the original colour. Only after that, decide if you want to proceed with removing the stain/lacque.
When you decide to sand the wood: use the finest sanding material you can find! And give it a try on the underside first, making sure that you get the result you want.
Or, fork out some money and have it done by a proper furniture restorer. For me, these chairs are worth their money.
Menno