Then you're probably not really using the Droid Sans Fallback font set, because they do handle these characters

.
(Also, not sure if the UTF-8 BOM plays nice with the Kindle reader, but it shouldn't hurt. At worst you'll see something weird as the first char. But just note that UTF-8 BOM is EVIL!

).
Also, not sure which font is used when you drop a .txt file on your Kindle, but I seem to remember it being the Sans Mono, not Serif or Sans, which might explain why it's not working, depending on how/which fonts you installed exactly. Convert it to MOBI.