Love.

*pick your own language*

MeCha

I suppose there's that old (untrue) one about how the Chinese character for 'crisis' is made up of the characters for 'danger' and 'opportunity'.

Heh. I just mean a general noun, not a specific one. Like, say I have a huge bushy tail and all the babe squirrels dig it, but at the same time it keeps me from having good balance and I fall off the larch (the. larch.) all the time. Simultaneous blessing and curse...

I was just thinking about that chrismear, that's what I mean, something along those lines, but true.

Not a foreign language, but I suppose you had thought of "Mixed blessing"?

Isn't this the origin of the phrase "White Elephant"? Because a white elephant is very rare and very valuable, yet it'll eat you out of house and home.

I think in portuguese, the word saudade is supposed to represent the longing of loss or something similar. The happiness of regret or some such?

zweischneidige, German for two-edged, can have the same connotation as in the idiom "double-edged sword."

A white elephant specifically denotes a gift given that the receiver finds useless. I don't think it applies to stuff that just happens, except in the loosest possible sense.

The concept that springs immediately to mind is the Tao. Or yin and yang. In Western terms, you have Fate (as opposed to fatalism), or Fortune. Something along those lines?

If you're looking for a word as such and not a concept, well, you're looking for an auto-antonym (aka contranym and numerous other terms). I googled around for some non-English ones and actually found one that comes close:

In Hebrew, lekales means both "to praise" and "to disparage".

And for Flo, here's Urdu's lāg, which means both "enmity" and "love" ....

A penalty kick. But that's English. Which is foreign to me.

Pandora's box?

Yah, Allah!

But a big bushy tail gives you better balance. I think someone just outed themself as having never been a squirrel.