"yIqIm! jIjatlh jIH."
Translation:Pay attention! I will talk.
jIjatlh means "[I] speak," so is jIH which means "I" strictly necessary? Would jIjatlh alone have been grammatical?
It is not necessary. It is completely optional and it is grammatical without it. Though sometimes it can add a little emphasis. This cannot be represented easily in English, but maybe something like "I (and not someone else) speak." In any case, with this sentence we are trying to show that it is optional and that the sentence basically means the same thing whether you have it or you don't. You seem to have learned that lesson. majQa'!
I'm assuming the translation is supposed to mean "Pay attention [to me]!", in which case, why is the original sentence yIqIm and not HIqIm?
Oh, I think I got confused about the prefix yI-, since it's used both for 3rd person singular and no object.
Glad you noticed that! Either is fine. The way it is written above is a call for general attention even though the person really is hoping that they will pay attention specifically to them. The general call to attention is appropriate and not incorrect. As you suggest, a command to pay attention directly to them might have been even better.