In some common implementations if $LANG is set to certain values, it will fail to match some ASCII letters. This is because not all latin character using languages put Z last in the alphabet.

Try this (you probably need to enable and generate the locale first)

    echo y | LANG=lv_LV.UTF-8 grep '[a-z]'

Locales in general should be considered a "trap", just look at Windows CSV separator handling, etc.