Not sure what I can do short of teaching you basic ROM hacking.
Games typically display text in one of two ways
1) Graphics. Common mostly in puzzle games and other games with very small amounts of text.
2) A text engine. Long form text like RPGs being where you find this, though you can find this on anything that only has text enough to say "press start" at the title screen.
For graphics you have to figure out where they are stored, also any compression (not so common in GB/GBC but not absent), and copy them. Sadly it will not be in a form you can type into unless you OCR it (nobody really does this) or manually transcribe it (a decent way to make sure you know your kana and learn a few kanji while you are at it).
Text...
basic way I describe it is when you were a kid you likely made a code like a=1 b=2 c=3.... games do something similar. In PCs the most common thing is/was ASCII
http://www.asciitable.com/ , though these days Unicode is the dominant concept and it is quite tricky at the higher levels there (basics
http://www.joelonsoftware.com/articles/Unicode.html ). The wind hex editor you got likely has functions called load table and mentions table a lot in the menus. The table is the list of encodings the hacker makes like 00=a and so on, nothing more. Making them can be tricky. Said hex editor is probably not the best plan either as they are not really designed for dumping scripts, though it will hopefully display it and that can get it done if you only want it to read.
For Japanese the traditional big two are shiftJIS (
http://www.rikai.com/library/kanjitables/kanji_codes.sjis.shtml ) and EUC-jp (
http://www.rikai.com/library/kanjitables/kanji_codes.euc.shtml ).
Games however are not required to use them and usually don't, at least on older systems anyway (the DS saw many games use a subset of shiftJIS). As Japanese does not have an order (a very powerful tool for Roman alphabet using languages is relative search), and also thousands of kanji it is often a right pain to figure out for Japanese. There are various things you can try and patterns that have been observed in the past (order it appears in the script, most common character first, it may match the orders seen in the various teaching papers,may match the orders seen in shiftJIS or EUCJP or something like that).
Oh and on top of all of that you still possibly face compression and other fun things as the GB/GBC is limited in power and there are things to work around it like substituting tables. Oh and pointers and markup and such.
You can learn a lot of this over a weekend, however to get to the point where you can walk up to any old game and expect to make some decent headway can take a bit longer.
I cover this in far more detail in
http://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ and it is the subject of basically every text or general ROM hacking guide out there.
http://www.romhacking.net/start/
If don't know if I have anything in Portuguese right now, most such work tends to be in the Brazilian flavour too.
