https://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ is more for the GBA and DS but the 3ds is fairly similar, and ROM hacking in general is fairly similar in base concept whether you are doing the first things to load tapes or a modern PC game released yesterday.
https://www.romhacking.net/start/ being another link in that.
You usually start with a tool that can pull the ROM apart if your system allows it (DS or newer, anything that came on floppy disc or optical disc). I don't know what we are suggesting for the 3ds,
https://gbatemp.net/threads/release-every-file-explorer.373615/ should be a start though.
There might well be some stuff in French out there as well but I don't know what we have for French flash cart and emulation sites, much less ROM hacking specifics, these days.
Generally for a translation though you will
Want to find the text. Hopefully it is as simple as looking at file/directory/extension names and sizes, including eliminating things based on that.
Want to figure out the encoding (if you ever did A=1, B=2, C=3 then similar things happen in console games, however they can and usually are entirely custom compared to PC where things usually pick from a list of known things
http://rikai.com/library/kanjitables/kanji_codes.unicode.shtml http://rikai.com/library/kanjitables/kanji_codes.sjis.shtml http://rikai.com/library/kanjitables/kanji_codes.euc.shtml). Relative search is a good tool in this.
Want to figure out any markup (bold, italic, value to indicate a given character is speaking), placeholders (it costs ??? to stay here for the evening/to buy this sword in this shop, custom player names and such) and other layout related concepts (end of line/new line/ yes-no value) but you can push this for a bit and is mostly finding something in the script that shows differently in the game and seeing what it is, and then experimenting with values.
Want to find the pointers for the text. Games don't read text like word processor documents and instead will have fixed lists of places to find things within the file, aka pointers. Think contents page of a book and works about as well as deleting pages or stuffing new ones in and expecting the original to suddenly stay matching.
Want to find the font. Your average Japanese game might include a weak version of the English alphabet but chances of it including nice accents, cedillas and whatever else are lower.
Compression might get in your way for this.
I have not looked at the 3ds Inazuma Eleven games (ew football) but did get involved in some of the DS ones. There were some quirks in those that might make it differ from the ideal translation case, and it might well happen for the 3ds too (timeline wise then they are not so many years apart that a whole new engine warrants being made and the DS was firmly in the coded in C camp so easy enough to upgrade that approach for the 3ds).
Realistically though start out by finding the text (again names of files are worth looking at) and altering some stuff in place so you can make a change appear on screen without the game crashing. Everything else you can build up to. Don't go trying to translate by matching sizes of text, especially not for French (there is a reason all those people doing Arabic hacks to games and doing right to left text the cheat way pick a French base, that being they have the largest scripts to use as a base for that).
