'Withered' has the connotations of aged, decayed, shrunken, dried, desiccated. It's not actually a word we would often use of flowers, which might explain why people didn't understand. (The pronunciation could also be an issue, since W and TH and R are all sounds that can be tricky with a German accent.)
It makes me think of the phrase 'withered on the vine,' meaning dried up and dead before it ever had time to grow or flourish. Also of a character like Miss Havisham in Dickens; or of the song 'The Last Rose of Summer':https://en.wikipedia.org/wiki/Miss_Havishamhttps://en.wikipedia.org/wiki/The_Last_Rose_o...
The practical difference may be that when a plant or flower is withered, it is so dry that it's essentially dead, unable to be revived.
'Wilted,' however, can just mean drooping or sagging due to lack of water. If you have a nice bouquet of cut flowers and forget to water them often enough, they will wilt; but if you then trim their stems and fill up their vase with fresh water, they may still freshen up and live. Or in the late afternoon in the hot summer, flowers or plants in the yard may appear wilted, but if you run the sprinkler in the night while the temperatures cool down, they may be fine in the morning.
Not sure if that helps. If you had a specific context example, or a German sentence, it might be easier.