I can't see the images.
font.bin is usually a good guess for the game's font but as you can see in the images in your earlier post there are games that have multiple files.
If you are lucky then you will be able to simply view it Crystaltile2 or another GBA format (the DS shares a format for most aspects of 2d) capable tile editor. Indeed for CT2 no need to extract it first unless it was compressed -- in the little DS window where you have all the file names if you double click on it then CT2 will set the location of whatever viewer you have up to that location (you also get a indicator in the status bar to say what file the currently selected location belongs to).
Presumably here you would have it on one of the GBA modes (the original game has a drop shadow so start with GBA 4bpp) and whatever size the game needs (can be anything -- seen some that are all standard fixed size, seen some that are rather odd sizes compared to most other tiles, seen ones that crop the tile down for display but is larger size in the storage, seen ones that do different sizes for each and every character...).
I and to the best of my knowledge nobody else either have never studied a variety of font.bin files between different games to see if they are similar to each other before, and have had some that were rather complicated as well.
Once you have a visible image you can shift to the edit window (or another program and use the same settings you just learned) and doodle in whatever accents you like. Overwriting characters in an English/European game can be tricky compared to Japanese where you likely have a thousand potential victims for your alterations -- many have had to overwrite punctuation not otherwise used or tweak their script (I am not so hot on Brazilian Portuguese but I know some characters are super rare and mostly used for loan words, if you can alter the script so they don't appear you gain a character you can play with), or possibly do things like double up characters on specific tiles if certain letter combinations occur together. If you have to add whole new characters then if it is not NTFR format or something equally extensible and understood then it is actually quite a considerable hack most of the time (enough that many opt to translate the Japanese version instead or another version with more characters already in it).