On: An ex-Googler’s guide to dev tools

[beagle3]

Not clear. Look at https://en.wiktionary.org/wiki/wallah#Etymology_2

wallah is arabic slang for "by god"

voila is french for "look there", meaning "just look at that"

They are totally different semantically interjections originally, yet one fits almost everywhere the other does, and if Wiktionary is to be believed, in Danish one is a spelling for the other.