Could someone please explain why the "Lach" in Modim Anachnu Lach is not "Lecha"... I guess this is some grammatical point rather than the use of the feminime "you"?
The "Modim anachnu lach" in davening is a quotation from Divrei Hayamim I 29:13. "L'cha" becomes "lach" because of the etnachta, which is a pause in the pasuk.
As avi answered, (1) Lach is an acceptable form for males in Biblical Hebrew. I think that one will typically find this in more polite speech.
More than that,
(3). It is an alternation in Biblical Hebrew for the pausal form. Just as Lemech becomes Lamech at an etnachta or silluq (or sof pasuk), so does Lecha become Lach in these positions. Prayer is based, often, on Biblical Hebrew, and indeed employs pausal forms. E.g. gashem, atta with a kamatz and stressed on the first syllable, and so on.
In terms of (2), that it is a way of referring to Hashem in the feminine, I doubt it because Modim continues she-atta rather than she-at.
I heard that we are grateful to the Ebishter's motherly side. We are thanking Gd for Mercy and so we appeal to the grammatically Feminine words for Gd (such as the Shechinah); as the above comment states. Remember Shabbat is the Queen. And since we were created in Tzelem Elo-kim as demi gods with creative powers, this must mean that everyone has both male and female components to our soul.