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!
A common Quarian phrase, from the video game trilogy Mass Effect, meaning "By the home world I hope to see one day." Exiled from their homeworld, the Quarians (an alien race known for their technological genius) have lived as space-nomads for about three centuries. Like the phrase "peace be with you", the phrase "keelah se'lai" can mean many things. But whether to express surprise, happiness, or frustration - for them to swear on their homeworld is the meaningful expression they have.
to an army waiting to massacre a synthetic race that drove them from their homeworld, although, ha-ha-ha, they claim to be peaceful now, a frustrated soldier says: "The enemy doesn't want to fight you! If you can believe that for just one minute, this war will be over. You have a choice. Please. Keelah Se'lai."