yvanspijk, 
Why is 'laugh' written with -ugh while it ends with an [f] sound?
It's because the spelling 'laugh' reflects how the word was pronounced in Late Middle English, some 500 years ago.
Click the video to listen to a phonetic reconstruction of how this verb evolved from 3rd-century BC Proto-Germanic to modern-day Standard English.
The Middle English to Early Modern English stages are based on the dialect of the region of London.
