Thank you so much LouBouha for your time

I found these lines in sys.txt

[fonts]
default="Noto Sans CJK SC, Regular"
defaultbd="Noto Sans CJK SC, Bold"
defaultit="Noto Sans CJK SC, Thin"


also thest files in ui_nextgen/font

NotoSansCJKsc-Regular.otf
NotoSansCJKsc-Bold.otf
NotoSansCJKsc-Thin.otf

because I am not expert enough in sys.txt syntax I tried to rename my files to same names(NotoSansCJKsc) and replace it with old one
But the application failed to start

I think we almost there if you please explain 3 more things

1- why font names in sys.txt are witting with spaces between them ("Noto Sans CJK SC, Regular") and if i put the whole names including extension well that be acceptable for example (xxxxxxx.ttf)

2- is ttf font format acceptable or I should convert them to otf

3 -do I need to add other font files for English language as well to switch between them