Often you hear quarians (from the Mass Effect 2 game) exclaim, "Keelah!" when something surprising or unexpected happens. Taking that into account, I would say that "Keelah'Selai" would most closely translate to the Dragon Age blessing of, "Maker watch over you," where the exclamation of "Keelah!" can be interpreted as, "Maker's breath!" in this case, "Maker" can be interchanged with "gods" or "fortune" or any other intangible form of fate accepted by each individual's preferred choice of religion or philosophy.
As for the literal translation it means, "May the spirits of our ancestors guide and protect us." or "May fortune smile upon us" in the quarian tongue.
Moe: I am going into the mountains of Death to rescue the princess from evil.
Attendant: Keelah Se'lai!