For example, a few years ago I grabbed a source tarball from somewhere, I forget what or where. It had the author's name in a comment, which included an O with dots over it. That was the only non-ASCII character in the source code. No matter what I did, both Eclipse and command-line javac refused to compile the source.

Finally I wrote a script to delete his name from every source file manually. It compiled flawlessly.

Then there's the time I found some text files with two characters of binary junk at the beginning, followed by completely normal text. Again, I forget what I was doing, but some program was refusing to process them correctly. It was something internationalization-related called the BOM. Eventually I ended up writing a script to walk a directory and remove the first two bytes of every file. (This can probably be done with dd and xargs on UNIX, but I was using Windows at the time, which means that something like this will require spending an hour or so in your favorite programming language.)

These experiences lead me to believe that, for bootstrapped USA startups at least, you shouldn't worry about a market outside the English-speaking world.

If you need to worry about junk like accented characters or moon runes (Chinese/Japanese/Korean characters), it means you're big enough to afford to hire someone specifically to address the problem.