'Lagom' was my favourite Swedish word and it doesn't translate easily. It means everything is fine or just right, but in a positive sense that the English doesn't convey.
As a cyclist in Budapest, I had much cause to use 'seggfej'. It translates as 'arseface', but means 'arsehole'. Terrible drivers, the Magyars.
'Marvellous' is a word that seems to have fallen out of favour, losing out to the awful 'awesome'. but is my favourite adjective.