"Han är den ende för mig."
Translation:He is the only one for me.
Shouldn't you accept enda too because in many parts of Sweden they don't even use masculine adjectives anymore?
Sure - we actually do. If you got that marked incorrect, it was a bug. It's been accepted for over two years. :)
Was it a "type what you hear" exercise? If not, then that was a bug. I can't imagine anyone from the team has removed a correct translation from the sentence.