How can I get my LCD display to display Chinese and Japanese characters?
Displaying Kanji or Chinese characters is way beyond the ability of an LCD that displays 5 pixel by 7 pixel characters.
To understand the limitations, take a piece of graph paper and draw out a set of 5x7 rectangles. See if you can define ANY recognizable Japanese or Chinese characters on your graph paper.
Maybe a couple of characters? (Then again, maybe not.)
You need a display with a lot more resolution, like an OLED graphics display. You are also struggling against a microcontroller that doesn't have support for Unicode. It's just not up to the job. You'd be much better off with a Raspberry Pi and a screen like a smart-phone color LCD screen. Install Linux and you can have native support for Unicode.
As others have pointed out, hiragana or katakana would be more practical, and there are some low-res LCDs that support those characters.