A word in Brazilian Portuguese that's really tricky to translate is 'saudade'. I'm a physiotherapist from Belo Horizonte, and every time an English-speaking friend asks me what it means, I end up talking for about five minutes.
Roughly, it means a deep longing for someone or something that you love and that isn't with you anymore, or maybe never was. It's a mix of missing, nostalgia and a kind of sweet sadness. You can feel saudade for a person, a place, a time in your life, even a food your grandmother used to cook.
We use it all the time. If a friend moves abroad, you'd say you're full of saudade. People end text messages to relatives with something like 'big hug, I've got saudade'. There are countless songs about it, so it's almost part of our national identity.
The reason it's hard to translate is that English splits the idea into separate pieces. 'I miss you' is close, but it feels quite light, almost practical. 'Nostalgia' is only about the past, and 'longing' sounds a bit dramatic or old-fashioned. Saudade is all of those at once, and, importantly, it isn't purely negative. There's a kind of gratitude in it, because you only feel saudade for something that made you happy. When I moved to Ireland for a year to study, I really understood it, because I'd sit on the bus thinking about my mum's kitchen and feel both sad and warm. I've never found a single English word that captures that bittersweet mixture.