I came across the problem a couple of times in my career as a freelance translator: I am working with a Japanese OS, but I mostly write in French (and its accented letters) as it is the language I translate into.
Sometimes I will open a software and realize accented letters appear garbled, preventing me from doing my job correctly (it happened to me in XBench, the otherwise great Notepad++ and the generally not great at all Microsoft’s LocStudio). After some research, I found out that the problem was with my system’s locale, which was set to Japanese. After changing it to French and restarting the system, the problem was gone: accents were displaying just fine
Here is how to change the system local in Windows 7. The process is similar for other versions of Windows, menu names possibly changing a little bit on the way.
The same problem goes with Japanese characters appearing garbled in software. If you have such a problem, switching to the Japanese locale may be the solution.
Since I use both languages, I opted to have one of my computers set up with the Japanese locale and the other one with the French locale. Fortunately, most of the jobs I do are not concerned by this issue, so I only occasionally have to switch computers to get things done.
Update
Please note that since version 3.0, XBench can display characters correctly regardless of the locale. However, this is a paid version of the once-free software, now offered under the SDL banner. The choice is yours!