TFT LCD(ili9486) Korean arduino mega(2560)

Other Hardware Displays
3

I suggest that you use <U8g2_for_Adafruit_GFX.h> which is available from the IDE Library Manager

#include <MCUFRIEND_kbv.h>
MCUFRIEND_kbv tft;

#include <U8g2_for_Adafruit_GFX.h>
U8G2_FOR_ADAFRUIT_GFX u8g2_for_adafruit_gfx;

void setup()
{
    tft.begin(tft.readID());
    u8g2_for_adafruit_gfx.begin(tft);                 // connect u8g2 procedures to Adafruit GFX
    u8g2_for_adafruit_gfx.setForegroundColor(TFT_WHITE);  // apply Adafruit GFX color
    u8g2_for_adafruit_gfx.setBackgroundColor(TFT_BLACK);  // apply Adafruit GFX color
}

const char utf8_msg[] = "由沙子制成的城堡最竟落入海中"
                           ;

void loop(void) 
{
    //u8g2_for_adafruit_gfx.setFont(u8g2_font_unifont_t_japanese2);  // use japanese
    //u8g2_for_adafruit_gfx.setFont(u8g2_font_unifont_t_chinese2);  // use chinese2 for all the glyphs of "你好世界"

    //u8g2_for_adafruit_gfx.setFont(u8g2_font_unifont_t_korean2);  // use korean for all the glyphs of "헬로월드"
    u8g2_for_adafruit_gfx.setFont(u8g2_font_unifont_t_korean1);  // use korean for all the glyphs of "헬로월드"
    //u8g2_for_adafruit_gfx.setFont(u8g2_font_10x20_t_cyrillic);  // use cyrillic for all the glyphs in "Привет, мир"
    //u8g2_for_adafruit_gfx.setFont(u8g2_font_inr24_t_cyrillic);  // use cyrillic for all the glyphs in "Привет, мир" 
    u8g2_for_adafruit_gfx.setFontDirection(0);
    tft.fillScreen(TFT_BLACK);
    u8g2_for_adafruit_gfx.setCursor(0, 30);
    u8g2_for_adafruit_gfx.print("Hello World!");
    u8g2_for_adafruit_gfx.setCursor(0, 60);
    //u8g2_for_adafruit_gfx.print("こんにちは世界"     ); // Japanese "Hello World"
    //u8g2_for_adafruit_gfx.print("你好世界");        // Chinese "Hello World"
    u8g2_for_adafruit_gfx.print("헬로월드");   // Korean "Hello World"
    //u8g2_for_adafruit_gfx.print("Привет, мир");        // Russian "Hello World"
    u8g2_for_adafruit_gfx.setCursor(0, 80);
    u8g2_for_adafruit_gfx.print(utf8_msg);

    delay(1000);
}

EDIT. Sorry, I ran this sketch on a STM32. I did not notice the Mega2560 i.e. AVR
The korean2 font is too big for an AVR. I have changed to korean1