Here is the full list of all 47 Learn Katakana!! (JP) trophies - there are 38 bronze, 5 silver, 3 gold and 1 platinum.
You have learned them all
You have learned "a"
You have learned "i"
You have learned "u"
You have learned "e"
You have learned "o"
You have learned "ka"
You have learned "ki"
You have learned "ku"
You have learned "ke"
You have learned "ko"
You have learned "sa"
You have learned "shi"
You have learned "su"
You have learned "se"
You have learned "so"
You have learned "ta"
You have learned "chi"
You have learned "tsu
You have learned "te"
You have learned "to"
You have learned "na"
You have learned "ni"
You have learned "nu"
You have learned "ne"
You have learned "no"
You have learned "ha"
You have learned "hi"
You have learned "fu"
You have learned "he"
You have learned "ho"
You have learned "ma"
You have learned "mi"
You have learned "mu"
You have learned "me"
You have learned "mo"
You have learned "ya"
You have learned "yu"
You have learned "yo"
You have learned "ra"
You have learned "ri"
You have learned "ru"
You have learned "re"
You have learned "ro"
You have learned "wa"
You have learned "wo"
You have learned "n"