Popups launched from a HTMLArea editor (e.g. to insert an image) only send the encoding in a meta tag, and not in a HTTP header. Now if the web server is configured to send a HTTP header for all pages with an ISO-8859-1 encoding, and the Moodle site in UTF-8, this gives problems : foreign characters in the popups appear garbled, as the HTTP header encoding overrides the one specified in the meta-tag (UTF-8) (note that this is the correct behavior, according to section 5.2.2 of the HTML 4.01 specification).
This problem doesn't occur with other Moodle pages, since they all send a HTTP header with the character encoding (in addition to the encoding in the meta tag).
The solution would be to add a php statement like "header('Content-type: text/html;'. current_charset()')" right after "include(config.php)" in all popups inside /lib/editor/htmlarea/popups.