In Russian we have two different word for “why”: the one asks for a purpose, the other one asks for a reason.

So you can not only distinguish between a purpose and a reason, but ask for either with just one word. Pretty efficient!

@frogstair почему asks for a reason, зачем asks for a purpose. Both translate to “why”

@uyouthe It's similar in German: "warum" asks for the causal reason, "wozu" for the purpose.

Though in practice, people are sloppy and use the former also for the purpose, just as in English. The logic is that reaching the purpose in the future is the reason why someone has started something in the past.

@frogstair exactly, that’s the actual meaning. But so many texts translate зачем to why

Hum... Why?

@Fast-Nop In contrast to "warum" the question for "weshalb" did not lose its focus on the reason. So we still have the two different meanings of "wozu" and "weshalb" in German. Nearly the same for "wieso", which also still has a (albeit more subtile) reason focus.
All in all this is a rather nice situation: We can ask for the reason, the purpose or whatever is the cause of what has happened.

Russia: We have two distinct words for "why"
Germany: Hold my weissbier

So, what's the difference between warum, weshalb and wieso?

Hebrew also has this distinction. Downside: no deliberately answering the opposite of what people are looking for in order to annoy them.

@VaderNT warum, wieso, weshalb (wes-halb) and also weswegen all are synonyms in real usage. Most often, warum is used while weshalb and weswegen are uncommon and sound a bit dated. Although they allow attaching subordinate sentences like "which is why", something that warum and wieso don't allow.

wieso is the only one that doesn't have a causal reply rhyme meaning "that's why":
warum? - darum!
weshalb? - deshalb!
weswegen? - deswegen!
But there is no "dieso" that would rhyme with "wieso".

So if someone tells you: do this and that and you ask back why, you use "wieso" to rob the other guy of the rhyming "that's why" move.

The actual alternative meaning is "wozu", asking dedicatedly for the purpose.