Most wooden log cabins are offered without external sills. The reason is of course that the wall thicknesses usually are about the same than the thickness of the window frame. So external sills are not needed to protect the wall beneath the window against rain, like in buildings with thicker walls. Another thing is when you have a wooden log cabin made of really thick round logs, of course.
So, for most wooden log cabins external sills would only serve decorative purposes.
However, if you would like to have flower tubs outside to decorate the windows these are much better placed at the wall below the windows, so they don’t cover the actual glass and thus reduce the light inside. For that purpose, metal hooks are much better. They are fixed at the wall below the window and carry a flower tub so that the tub is not visible from the inside but the flowers are.
Additional insulation of the wooden log cabin increases the wall thickness
If you would like to use your wooden log cabin all year round then you might put an additional layer of insulation onto the walls. Wherever that is done, a window sill is most probably advisable. So if you add insulation on the outside an external sill below the window sills will protect the double wall from rainwater flowing down the windows and provide some cover for the new external layer for the rainwater to drop off of the external sill in a safe distance to the wall.
For the inside, the problem seems not that pressing, but as insulation is mostly added to a wooden log cabin because a heating is installed, there will most probably be condensation on the inside of the windows, and this means a window sill is needed to keep the insulation dry. Condensation water at the windows could possibly only be avoided by insulating the windows, too, and using triple glazing while trying to keep the inside as dry as possible.
Internal window sills in the wooden log cabin provide a practical and a decorative element
A window is always also a decorative element in a wooden log cabin. Curtains somehow look better with a window sill and then there is the option to place some decorative objects on it.
From a more practical view, the window sill provides space to store things. For gardeners, for example, it might be practical to have a space to put frost-sensitive plants on during the cold season. Some gardeners also like to start with the propagation of the plants for the new gardening season as early as possible. In these cases, it might be useful to have really large window sills where you can place even 2 to 3 seed pans on.
These kinds of window sills can easily be made by most people with a little DIY experience with the help of a wooden plate and a couple of metal carriers from the local hardware store.
For gardening purposes, you should, of course, use some water tight material like a kitchen-top or treat the wood yourself with a water resistant lacquer or mask it with an adhesive film.