I get things mixed up too as i am self taught in all the languages,,, well me and TV.
Norwegian and Swedish i some times mix, Spanish and Italian too.
The main reason i think are i don't really use those languages much, and Spanish and Italian, well saying i speak those might be stretching the truth a little.
But i would have no problem at all functioning in Norway / Sweden / Germany and any English speaking country, some countries i would be totally lost in, even with the flair for languages my brain seem to have.
Of course i am not 100% in any of those languages, but its so good that if i miss or don't understand a word now and then, i can deduce from the context what is meant.
German and English was mandatory in school back then, but as i already spoke it better or as good as the teachers then paying attention in those classes did not really work for me, so i spent many of those outside the door or at the principals office.