Directly after the phrase "Hashem yimloch l'olam va'ed" * in the paragraph of Az Yashir in the siddur, the phrase is translated into Aramaic: "Hashem malchutei ka'eim l'alam ul'almei alma".
Why is this so?
My personal speculation would that be that the line is left over from when the majority of the Jewish world spoke Aramaic, in order that the person praying would understand what they were saying. I'm not sure about this, though.
* To understand why the phrase "Hashem yimloch l'olam va'ed" is repeated twice, see here.