Kale is a "feed in moderation" item. kale has a high calcium content and only half the *oxalic acid* of dandelions, making it a potentially nutritious food. Unfortunately, like all the brassicas, it is also high in *goitrogens* (which interfere with iodine uptake, resulting in thyroid problems). However, kalealso has a high iodine content, which lessens the goitrogen effect and it is therefore acceptable to feed to your tortoise in moderation, especially in winter with non-hibernating tortoises, when fresh weeds are scarce
@Tom has a very nice diet sheet for russians. I can't remember it now (go figure!) But it has a lot of great food choices for russians.
Kale can be healthy in small amounts... I use it as a treat
"Staple" doesn't mean you feed every day. It just means it's an ok part of a varied diet. There is no ONE thing that should be fed every day, every day. If anything, the staple would be weeds and plants. Then for variety, different things from the produce section of the grocery store.