The most authoritative analysis of what markets Quora could most usefully move into is by Josephine Stefani: Josephine Stefani’s answer to What other languages should Quora support?
The upshot is, I’m guessing Quora picked German and Italian because it is probably easier to actually set up shop in these markets. But if it really wishes to adhere to its well-worn slogan of ‘sharing and growing the world’s knowledge’, I think it should have a look at other parts of the world that really need it: Arabic, Russian (maybe), Portuguese, Japanese, Korean, Indonesian …
My own answer identified Arabic and Russian too, but Josephine did an awesome job of it.