Problem with non-utf8 handline

Normally, grep will print the small case f.
$ export LC_ALL=C
$ echo f | ./grep '[a-z]'
f

However, with some locales it does not:
$ export LC_ALL=ru_RU.UTF-8
$ echo f | ./grep '[a-z]'
(nothing printed)

As you see, grep in ru_RU.KOI8-R locale doesn't grep.