So yeah get the ROM/iso and if it is one you can unpack then do so, if not then the next step is much the same as you need to find where the data is and what the hex there represents. That is one sentence but can still represent a great many hours of work.
If you want to translate a game you have to know what you want to translate it into. This is mainly as the original devs might not have thought to program it in such a way that you can easily do that -- in general the text might be longer, it might be read from right to left, it might have characters of all sorts of widths and it might not even have the pictorial representations of the language in question (not everybody uses the Roman character set after all, and many have weird and wonderful additions to it). It is not mandatory to sort all this at this point but it is highly advised you at least have things in place to go for it as and when. Hopefully said changes are just changes but if you face space issues (can still happen on iso files as things get transferred to memory or have to fit on screen) then you also get to put something in place to sort those.
Anyway you can use a hex editor to edit text as a hex editor will edit any data in a file -- it is what it is there for after all. However for anything more than a menu or minor typo fixes it is a nightmare so you instead get to figure out how you want to edit the script.
So I assume you are versed in table files, if not then they are small files that hackers make that list how a game changes hexadecimal into text, they need not bear any relation to anything used elsewhere in computing (things like ASCII, unicode, ShiftJIS, euc-jp and so on)
Your translator is typically not going to be as versed in computer encoding and file formats as you so they will be very happy with a nice plain text file to edit. A nice plain text file is a nightmare to put back into a ROM* so we have to find some kind of middle ground, or make a program that can view the text but still allow you to fiddle with the thing as it will likely end up looking in the ROM (the right screen placement, colours, line size...). Said program can be an awful lot of work to make though so few do that if they can help it, indeed it is probably the main barometer of when a game turns from a game people know a bit about to having its own dedicated hacking fanbase and about the only thing I would rate there without serious text editing capability is fire pro wrestling or something like megaman which has no text most of the time for the older entries.
To help with this though a variety of ROM hackers have various means of dumping and insertion. In the English speaking world and among those that know ROM hacking from here, whirlpool, zophars and such sites then most will look to things like Atlas and Cartographer. If you came up through China then you will probably be aware of crystalscript (from the same original author of the crystaltile2 program which inherited a lot of the functionality, though nothing I care to use for anything more than rough search and fiddle), know some Russian and it will probably be kruptar and oriton (though these have some folllowing outside there). As text is a hugely variable affair then any generic program probably has lots of functionality, or is near useless for anything but the original game/format the hacker made it for. Atlas and Cartographer being a good example of the former (they are hard to use but once you get it then they are great, until they meet something really really custom) and kruptar and oriton doing worse but being far easier to pick up. In any case both of those are quite complex so if you have a vague clue about programming you will probably want to seriously consider your own custom program to handle it as well.
*a nice plain text file probably uses 0d0a to signal a new line, very few games will do that so first you get to fix that. So far so boring, now add in variables (it costs ?? a night to stay at this inn), character name placeholders, any binary that might start to indicate what character is speaking and you have yourself a nightmare if you are doing it by hand to restore a plain windows compatible text file into something you can use (though if you thought ahead then you would hopefully have made said problems into XML tags or something so you can easily convert back). This is also why most we do not point people at places like gamefaqs that have loads of game script dumps as they are all pretty much useless for ROM hacking purposes.
Oh and even with all that you probably still have pointers to solve -- games do not tend to know when something ends and will have to be told everything, possibly even when to start a new line. This is probably the biggest annoyance in text (and many other types) or hacking. As the pointers the game reads will tell it to go fetch this many bytes at this address then it will, however if you have shuffled everything forward by 10 bytes as you made the last line longer the game is going to come unstuck. OK so you fix that, however everything following it is also 10 bytes out so you get to sort all of that. You are not necessarily going to edit one line though so everything is now plus or minus however much you change it by plus the differences of everything before it.
This is probably the chief problem with using a hex editor to edit text, and why people will try to make things the same length, or make it short and pad it out to the same length.