אנכי אנכי הוא מנחמכם ... מי את ותיראי ... (ישעי' נא-יב)
After Hashem declares: “I, I am the one who comforts you,” Yeshaya HaNavi continues to ask "מי את ותיראי" – “What ails you that you fear?” The question, while directed at Klal Yisroel, is nonetheless asked in לשון נקבה - the feminist linguistic, as opposed to the masculine style. Why is that?
R’ Binyamin HaKohen Vitali zt”l (Gevul Binyamin) explains that Chazal teach us that all Shiros are done in לשון נקבה except for the final Shira in the times of Moshiach. This is because in the aftermath of each exile, the Jewish people created a new life for themselves. True, they may have become more righteous in the immediate aftermath, but that inspiration always seemed to wear off eventually. So, just like the female which has the ability to give birth to a newentity which may stay pure or choose to falter, so too, the Shiros sung by Klal Yisroel in the aftermath of their salvations was sung in לשון נקבה since their newfound spiritual awakening could last a while only to falter eventually. However, when Moshiach arrives, the Shira will be sung in לשון זכר to emphasize that the inspiration would last for eternity.
While Hashem comforted Klal Yisroel in the aftermath of the Churban, Yeshaya HaNavi knew all too well that Bnei Yisroel’s newfound closeness would probably be short-lived and so he chose to ask his question in לשון נקבה as a way of hinting to Klal Yisroel to hold onto that burst of inspiration for as long as they could.