Trouble with Japanese Games

[apraxiumRum]

Hello! It is my first topic and it is about translating so hear me out...
I am currently trying to translate a Windows95 old Japanese game called Dragon Slayer: New Heroes Legend.
I've managed to get to the game files and find text and conversations to translate with Notepad++...when troubles started.
Turns out the game gets upset and crashes upon injecting English letters in its files, probably a problem inputting English characters in an all Japanese game.
Can you help me solve this problem? I figure that I should input English letters that are within Japanese CharSet.
Your inputs are appreciated!

[FAST6191]

" translate with Notepad++"
Tends to be a bad plan even with actual text files. Have you confirmed your script files are just text? No pointers, file headers, extra binary formatting or the like.

If it is encoding... possible for a PC game and especially of that vintage and for as many ports as it apparently saw (for a console game we see a lack of Roman characters/ASCII support all the time) then maybe try using the http://www.njstar.com/cms/ or JWPCE (original download seems to be down but mirrored in plenty of places).
Both of those support using the Roman character parts of the likes of shiftJIS ( http://www.rikai.com/library/kanjitables/kanji_codes.sjis.shtml ) and EUCJP and thus are popular for said console hacks where the font/encoding might have those and you only need a couple of lines of it*. In JWPCE you will tend to have a J on the top button bar which puts it in that mode for a normal keyboard, forget where it is for njstar but it tends to be a mode you select in any programs like this.

*start translating an entire script for a RPG or something and you start getting memory and space issues, for a menu translation or something low text you can usually get away with it. For a PC game you can probably do a far longer script so give it a go if you want.

[Psyklax]

I agree, Shift-JIS has a Roman alphabet plus Arabic numerals, but this is also assuming there is no other data in there that you might have to mess with. Being a Windows game, I have no idea.

[apraxiumRum]

Thanks to your kind inputs, I made a series of editing in files using a combination of HxD Hex Editor and Notepad++ (turned out I had to make a key table for English letters in Shift-JIS Hex values and tune the encoding), I figured I should first edit the game's exe file first (It is a pain already). So I tried playing the items because it looked easy....
I've managed to change (harmlessly) the "Herb" item using the method I mentioned.(Using the Camera because the game always goes fullscreen)

Notice that レスの葉 and Herb have 4 characters needing 8 Blocks in Hex Editor
However  the same can't be done with "Torch" or たいまつ because
I tried writing it normally and "inserting it" thus increasing the file size, when this happened:

As FAST6191 mentioned, it is the memory/space issue. The exe file completely changes and is rendered unusable. Notice it stayed itself when I changed "Herb"

I consider this an improvement, since I was trying and failing at changing the "Slime" enemy file.
Whatever this problem is, it is going to be present in EVERY LINE OF CODE FOR THIS GAME. If a solution is figured out and mastered, the translation for this game might become a possibility one day.
I thank you again for your ideas .

[Psyklax]

Well yeah, if you add bytes to a binary file, it throws everything out of whack. Herb worked cause the original word was four characters, but Torch didn't because it's five characters. By adding that extra byte or two, the whole file is ruined. I know how I might go about fixing it on a console ROM, but as for a Windows executable... No experience in that area.

[Jorpho]

Resource Hacker is the first thing I would try.
http://www.angusj.com/resourcehacker/