Why 你在笑什么？, why not 你笑在什么？ In English, you say

What are you laughing at?
I'm laughing at this situation.


Here, the preposition "at" refers to "this situation" and "what", respectively. In German it's more clear, since the preposition "über" is always before its target:

Über was lachst du?
Ich lache über die Situation.


Same in French:

De quoi ris-tu?
Je ris de la situation.


In Chinese, the same question would be (according to Duolingo and Google Translate):

你在笑什么？


To me, this sounds like

You about laugh what?
Du über lachst was?
Tu de ris quoi?


instead of

You laugh about what?
Du lachst über was?
Tu ris de quoi?


Why is the 在 before the 笑, instead of before the 什么? Is that just "because it is", or am I missing an important point here?
「在」in「你在笑什麽」is not equivalent to English at in what are you laughing at?. To demonstrate by analogy:


你在吃什麽 - what are you eating?
你在做什麽 - what are you doing?


「在」is actually equivalent to the suffix -ing in laughing, eating, doing. It is English which grammatically requires something as a target for the verb laughing; this requirement is redundant in Chinese.


  你在笑什麽
  
  Literally: You (你) [action in progress / -ing] (在) laugh (笑) what (什麽)?


Perhaps it is easier to see what's being omitted in English.


你在笑什麽 - about/at what are you laughing?
你在吃什麽 - what [food] are you eating?
你在做什麽 - what [action] are you doing?