Why does the fifth commandment end with a female "to you" (singular, 2nd person, female) while all other forms are male(singular, 2nd person, male). This is the only time that a female form is used in the ten commandments.

The use of the softer feminine “lach” for “you” in place of the masculine “l’cha” is found very often throughout the Torah as a type of “poetic license” for the beauty or perhaps “softening” or even variety of the style, see for example also: Shmot 32, 34; 33, 5; 33, 14; 34, 17; Vayikra 21, 8; 25, 15; ibid, 16; ibid, 39; Dvarim 1, 21; 3, 26; 6, 18, etc. etc.