"How can I learn the tile value in the rom? Let alone for all the Japanese characters so I can transform it to CaveSpeak?"
As in the value the game encodes to eventually display that tile/character?
That's the trick really.
For Japanese you then lack a lot of the techniques we can use in European languages like the really potent relative search, some of the language based tricks (space is the most common character and rarely more than 8 characters apart, RSTLNE being the next common characters, every word has a vowel or Y, the phrase " the " likely being in there somewhere) and the like. I am sure there are some for Japanese but both knowing them and figuring out how they would show themselves in a game is a different matter.
Do you have part of the script for the game? If you know what some things mean you can infer what else is in the text.
Equally if you have the script then you can change it and figure out exactly what the game does with a certain value (or values if you want to speed things up a bit; I usually use this to find odd bits of punctuation that a game may seldom, or indeed never, use).
Japanese characters have no set order, especially the kanji. Orderings seen in the wild include some of the language based ones (same as dai kan wa jiten, moji ordering, same as whatever educational reading list was popular at the time or the dev's school days), same as a given encoding (shiftJIS, EUCJP sort of thing https://www.sljfaq.org/afaq/encodings.html
), most popular in the game, same order as the tiles, first character used in the script/game, reverse of any of those, same as encoding used by same dev/pub in another game and the list goes on.
You can do an assembly driven workup for it. It will get you what you want (you will literally be watching the game fetch characters and learning how it does it) but it will be a lot of work so most only use it in extreme circumstances. For text this usually means fighting with some (han)dakuten table swapping script. On the other hand a follow through of that will a) put you in a good position to start adding characters, doing an 8 bit conversion or even a variable width font and b) might give you a good clue should one of the previous orderings be in play. Technically it might even give you a simpler means of matching encoding values to tile location/order but that is being a bit too optimistic for my taste -- it will usually be clouded in a bunch of instructions, not just some kind of hexed up mathematical table showing the relationship between encodings and tile value like we might see for a stats table in a sports game or RPG or something.
A lot of the time the devs, being people, will take the lazy route and not reinvent the wheel or create something too obtuse (there is nothing stopping them from cycling out tables every other screen and having them all be so very disconnected from anything else, we see individual aspects of that at times) so there is that going for you.
"How one can construct a table for 8x12 tiles to use in programs like WinHex?"
Same way you do it for anything else. Tiles aren't particularly special. Hopefully it is the same as a known ordering or you will be spending a lot of time with the manual character entry of something like tabular.
By the way if you want a nice tile viewer/editor capable of dealing with custom tiles (so many are limited to 8x8, 8x16 or 16x16) I tend to suggest crystaltile2. I normally use it for the oddities on the GBA and DS but it supports SNES too.