I'm really not sure how or what to error check. But I think if might be better to check that the conversion from string to number worked, to prevent someone from entering "four" instead of "4".

> Maybe this should check for sanity (i.e. greater than zero and less than some sane value, say 10). 

For checking the interval, if a language doesn't use thousand separator, a very large number would be expected here, I guess.

I hope the translated string description: "For large number, group digits by this many. European languages use 3 (group thousands)", will be clear enough.