Translation:I expressed everything with one word!
I guess, it was something from that list: https://cs.wiktionary.org/wiki/Kategorie:Vulg%C3%A1rn%C3%AD_v%C3%BDrazy/%C4%8De%C5%A1tina
Not sure how native Czech speakers feel, but from an English speaker, I do think "I said everything with one word" should be an acceptable translation. It may not be literal but I think it communicates the same meaning as "I expressed everything with one word" and is far more natural, the latter sounding fairly stilted.
Well of course in common speech you don't use such words as vyjádřit every day, but this exercise uses this word and you should translate it.
How can you know that the speaker of the original sentence is referring to saying something and not to writing something? Expressing is more general and not the same as saying.
And yes, you can use say figuratively evenfor writing, it is the same in Czech, but still I feel it is not the same.