In school, the language teachers should know & teach all the words in various languages that other languages don’t have words for. Is there a book or website indexing and translating/defining these words in English?
If we’re not going to do English as the universal base, may I recommend Esperanto?
It would give us so much insight into the way different cultures think uniquely about things. And the way they also think similarly to other cultures.
On one hand, I would want one that has every single language’s “words that other languages don’t have words for”
and then (on the other hand) perhaps a separate dictionary that has words that every single language has words for 
I am betting that there is a gradient between those extremes. So perhaps I would want a dictionary that has every single word ever, even the urban dictionary type words, and each word lists how many languages use it, and (separately) how it translates into other languages. And I would also like that dictionary to track the word usage over time, and its geographical (or other context) migration/frequency over time. Is that too much to ask?