The three obvious solutions that spring to mind:
Normalise the case before using a String as a key (not Turkish locale works differently from the rest of the world).
Use a special object type designed to be used as key. This is a common idiom for dealing with composite keys.
Use a TreeMap with a Comparator that is case insensitive (possibly a PRIMARY or SECONDARY strength java.text.Collator). Unfortunately the Java library doesn't have a Comparator equivalent for hashCode/equals.
