Hi;
hoping it's not an issue to reply to such an old post. But it is now years that I have this issue in XnViewMP, and that I wanted to file a bug about it; but, as always, as I had found a workaround, I postponed the writing of this message.
Indeed in XnViewMP accentuated characters are not managed as in all other software. Characters like â, ô ë... so to be able to type them in IPTC dialog for instance, you can paste them from elsewhere, then they will be displayed properly, but you can't type them directly in XnViewMP. "Direct" characters like à or é on French keyboard, are directly properly displayed in XnViewMP, but characters which need a composition key before issuing the final character, are displayed without the accent.
(On a French keyboard for instance, à or é or ù have their dedicated key, they are what I call "direct" here, and are thus properly displayed in XnViewMP. But so to write ô or â or ë, you need to type first the "^" key, and after that the key of the letter you want to put this accent on. In XnViewMP, this "accent compose key" is not managed, just as if you wouldn't have pressed this key, and â is displayed a, as all other accentuated characters composed this way. Though, if you paste a "â" that you copied from another software, it does be properly displayed in XnViewMP, therefore it means that it is only the "accent compose key" which is not "seen" by XnViewMP; the accentuated character itself makes absolutely no problem for XnViewMP.)
So to precise:
• It is the case with XnViewMP on Ubuntu (in my case, Ubuntu Studio 24.04 (which uses KDE Plasma desktop), and it lasts at least from Ubuntu Studio 18.04, which was using Xfce desktop);
• Other "compose" methods have the same behaviour: ie using iBus, one can compose Unicode characters like the dot used here (•) by sending a compose combination key (in my case CTRL+U), and then typing the Unicode code of the character (in the case of the dot I use here, &x2022, thus I make the key combination "CTRL+U 2022" and I get the dot). In such cases also, if you paste the character from elsewhere it's OK, but you can't type the character in XnViewMP.
• The behaviour is general within all XnViewMP: in any dialogue where you type characters, such as IPTC window; but also when you add some text in an image, or when you type the name of a filename — whatever you want to open or save...
• This behaviour does not occur in XnView in Windows, but I didn't make the test in XnViewMP in Windows.
And at last: many, many thanks for XnView

!