This problem seems to be in all versions XWiki (at least 7, 8, 9).
The feed: https://www.editions-tissot.fr/resource/rss/xml/conseils_tissot.xml
displays with bad characters as shown in the screenshot.
Firefox or Feedly are displaying this feed without any problems.
- relates to
XWIKI-14488 Add ability to specify the encoding to use to read the RSS feed for the RSS Macro