Katakana is the writtewn alphabet used for loan words, but you still pronounce it in standard Japanese pronounciation. So Orange Juice in Japanese is not "Orange juice" it's "Orenjijūsu". Cheeseburger is "chisubaga". Most of the loan words are English, but theres some interesting ones. "Part time job" is "arbaito" from the German "arbeit".
so, "I like orange juice" in japanese is "Watashi wa orenjijūsu ga sukidesu", it's not "Watashi wa Orange Juice ga sukidesu". Technically. In reality, since the original word and the loan word are so similar no one is really gonna be confused by it.