Couples in polyarmourous relationships often openly talk about who they date and how it affects their lives. Compersion then is the happiness experienced when your partner has found another partner and lives happily with them (in addition to you). [https://en.wiktionary.org/wiki/compersion](https://en.wiktionary.org/wiki/compersion)
This is technical jargon. There’s plenty of other examples but I’m not sure it fits the spirit of untranslatable words
Compersion is ability to feel happy when someone lives a happy situation even if you have no part in it. It is not exclusive to polyamourous relationships even though it is frequently associated with this kind of relationship because it is more frequent there. It can be you being really happy you significant other had a really great day without you.
Afrikaans has Compersie
Albanian’s Kompresim
Amharic’s ማመዛዘን
Means the same thing in Italian to