Why does the spell checker split words containing accents/umlaut/”special characters”?
Because your system is not configured to support non-English text: Claws Mail must be run with one of LC_ALL or LC_CTYPE environment variables set to a value representing your language. For example, if you want to be able to spell check French, you can run Claws Mail with “LC_CTYPE=fr_FR@euro claws-mail”. As French character set is iso-8859-15, you will be able to spell check all other European languages with this setting. You can set LC_CTYPE in one of your startup scripts to avoir setting it at each launch. For example, for an English locale supporting ISO-8859-15, you can use the Irish locale and put “export LC_ALL=en_IE@euro” in $HOME/.bashrc. See the locale man-pages (‘man locale’) for more information about locales and environment variables. See the man-pages of your shell for the startup scripts used by your shell. I have no idea on how GNOME or KDE desktops set locales.