My initial plan for finding out about the current locale is that the program will, at start up, look at the LC_CTYPE environment variable. If that variable is defined and contains the substring "UTF-8" or regex-able variants thereof (like "utf8" on Linux), then everything is fine. If not present, the program prints a warning message to the user suggesting they set the locale to a UTF-8 locale and provides an example of how to do that. If the locale is not set properly, the program still functions, but of course any UTF-8 encoded data will not be displayed properly on the terminal.
(Of course, even if a locale *is* set to a UTF-8 locale, it doesn't guarantee that UTF-8 data will be displayed properly because (1) glyphs still may not be available in the fonts on the system (2) the terminal may not handle the script properly (i.e., when I last checked, xterm didn't handle Indic or RTL scripts)).
I haven't much exp. with UNIX but for Multilingual apps this approach will be work without any problem. Fortunately in web env. (where most of time i worked) browser support UTF-8 all we need to do is to detect user lang. pref and render data in utf-8. But i believe above approach will work fine except limits you mentioned already about fonts and capabilities of user client (xterminals).
Asif
