Open the folder "Flower_Princess_Elulu_Version_1.98_Base_Game\translation/ENG", open inside the folder file "trans_scenario.csv" (and other files that also have lines, but they are less) in any text editor and start translating. For example, if you translate not into English, but into French or German, then instead of English lines you insert your own version. On this
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you can see how it works, from about 10:45.
Translate Japanese lines in the same way, only after the original line and the translation there should be one tab character.