Your English is clear enough. I have not got the game to look at so I will do the general case.
Replacing tiles is usually far easier than adding new ones, especially on older systems. If there is any way you can avoid adding things then do it; non English languages tend not to use the full alphabet for those languages so if you need one or two extras for something with accents or something then see about avoiding those in the script. If it is one of those rare GB/GBC games that has both upper case and lower case letters see if the script you are editing uses all of them (probably not many sentences starting, names or similar that use the letters that score highly in Scrabble in your language so might not need all those capitals).
You can add new ones though.
Three things to consider, maybe four.
1) Finding space in the ROM itself. This can be easy or can be hard, and for older systems like the gameboy then you don't necessarily have all the ROM available at once; see mbcs aka memory bank controllers. http://bgb.bircd.org/pandocs.htm#memorybankcontrollers
. They expand the space the game has at the cost of having to swap back and forth which can be done by hackers but is further work.
2) Memory issues. Graphics won't be read directly from the ROM but usually into RAM first.
3) Making the game aware of the extra graphics to grab them into memory and use them as well.
This is two things so why it says maybe four above.
a) For normal tiles then the order they are on screen is usually controlled with a so called map.http://bgb.bircd.org/pandocs.htm#vrambackgroundmaps
b) Most text on almost every system going is also done as part of the bg/backgrounds setup rather than as objs/sprites. However as text is inherently variable then most games have their own text engine that fetches tiles, modifies the VRAM containing things, decodes text to fetch the tile and more besides. You get to edit this. In later systems you can possibly tell it to grab a bit more data fairly easily and the rest is just maths (if text = 30 then fetch tile 21, if 31 then fetch tile 22 sort of thing), on older stuff where they had to get a bit more creative to fit it in then it gets harder. All those things mentioned you possibly have to edit, which usually means assembly coding, on top of finding suitable space in the bank and then in VRAM.