In Spanish.
Sobremesa, the period you stay at the table after lunch or dinner having a coffee or some drinks while chatting.

Aprovechar, is basically the opposite of to waste. So depending on the context is to take advantage of something, to make the most of, make a good use, give a second use or reuse leftovers for something else.

Does it count if English just steals your words?

Schadenfreude, Zeitgeist, Weltanschauung, Kindergarten.

Russian is more productive than English, so there's a bunch of "we just smushed a bunch of prefixes and suffixes on it" words that don't have a great English equivalent, like недоперепить -- to drink quite a lot but not succeed at drinking too much. I don't think we properly have листопад which is the equivalent of "snowfall" but for autumn leaves. I really miss успевать -- to successfully have enough time to do something. There's also кипяток -- water that has been boiled but then slightly cooled, like 200-205 degrees F. The canonical examples are тоска , which is a kind of melancholy longing, and авось which expresses optimism but I basically never heard anyone actually say.

Shaka, when the walls fell

Doch

There exists no english equivalent for doch, it has multiple functions, the most known one being an expression that emphatically negates a previous sentiment.

"Does not!"
"Does too!!"

Which created one of the most long lived german memes.

"It should not be that way, but it was."

But doch can do so much more.

In JavaScript, this.

Guten Appetit! (German term equivalent to "cheers" for drinking - but for eating.)

Gezellig in Dutch. Used to describe a situation that's cosy and fun togetherness. Can also be used to describe a place that looks inviting (to have that good time in). I guess 😃

Sisu. Its a mix of courage, determination, resilience, keeping your nerve, doing a scary or difficult thing even though you know that odds of succeeding are low. Its also done quietly, no complaining or boasting about it. You can mutter some curse words quietly to yourself.

Closest English word for it might be "guts" or "grit". But those don't really explain it completely.

Finnish.

German: Tja

Can mean nesrly anything from schadenfreude to deep empathy

Kindergarten

Fjøs is where farm animals are kept. "Barn" is more general and translates to "Låve", while Fjøs doesn't really have an English translation.

Nisse is a mythological creature that is part goblin, part santa, part Gnome, but not really either of them. They're usually benign and pretty cool.

Hygge (noun) /hyggelig (adjective) means having a cosy, lighthearted or fun atmosphere, usually in company of others

EDIT: By the way, Nissen lives in the Låve

お疲れ様 (good job+"you look tired"+"I an acknowledging this and presenting recognition of your effort")

Keigo modifiers. ご覧 vs 見る. They both mean "look". One is, somehow, more polite.

Kansai dialect. しばいたろか ("wanna get smacked?" but it's playful) or まいど ("thanks" but also "nice to see you again" and "hello" but only in specific contexts between specific people. Mess it up and you'll probably hear the other Kansai term previously described.)

German: Konjunktiv I and II. Most people first learning German will hear the Konjunktiv II in its usage similar to a subjunctive in english ("Wenn das so wäre" - "If it were that way"), but it does so much more. You can switch full sentences to the Konjunktiv II, but that immediately implies that you doubt the statement being presented. For instance, if you say "er hätte es vergessen", you're essentially saying "he allegedly forgot it, but I really doubt it", just by using the Konjunktiv II.

Then, there's the Konjunktiv I, which is similar, but instead of voicing your disapproval, you're staying completely neutral. Taking the example from before, it would be "er habe es vergessen", with which you're saying, "he allegedly forgot it, and I neither support nor disavow that statement" - again, all only with a change of verb form. It's wonderful when you're relaying something someone else said without adding your own evaluation.

Dreich - there is no direct English equivalent. It's weather that's wet and cold and dark and grey and unchanging. It can also mean depressing and tedious for obvious reasons.

"Kombinować" very Polish word for a very Polish a activity/philosophy. To make things work / get something done / take care of something in way that work for us, but are not the expected way to do things (though, 'kombinowanie' is often actually expected). It might be doing a job using the tools that are available and not thise that should be used, it might be a genuine invention, It might be using your contacts for favour (like getting a doctor appointment), it might be plain cheating.

A little bit like 'hacking' and also can be a good thing or a bad thing.

"Wykombinuję coś" = "don't worry I will take care of that" "Co za kombinator" = "what a cheaty bastard"

Javertus. (Danish)

It's a thing that sticks out, and you don't know what it is, but it has the function of sticking out. Like a naturally occurring bulgy peak or other unknown long shape coming out of something else.

It's most often used as slang for penis, but otherwise it might be found on trees or wooden objects.

झूठा खाना/पानी (Jhootha khana/pani)

Food/water that is cosumed by someone else and hence is unsuitable for eating/drinking. Especially for religious or devotional rituals.

In the similar vein, https://en.wikipedia.org/wiki/Prasada

Bukkake.

I'm having real trouble thinking of any in Estonian that truly don't have a good translation, there's only one I can think of on the spot like this.

Midagi läbi ussimunni ajama - Literally translated: to drive* something through a worm's dick. Commonly used in car repairs and other similar contexts: Some space is super narrow and you need to put something it through exactly at the correct angle. Can be used with "goes through" as well (käib läbi ussimunni), but even then it's generally still describing some sort of activity, not someone literally walking through a tight spot. I can't think of an English expression that's truly equivalent, though I could just be missing something.

* ajama itself means to drive as in "drive insane" or "steer a herd of cattle", not "drive a car". But also "sitta ajama" = "to bullshit".

Lanche, the meal between lunch and dinner

Chez

It’s a preposition meaning “at the home/house/place” but can also be use figuratively. Really super cool.