Fix double "<application>"
Maybe:
<!ENTITY app "<application>Klotski</application>">
Should be:
<!ENTITY app "Klotski">
In the translation po file i see:
#. (itstool) path: sect2/para
#: C/index.docbook:238
msgid ""
"When you start <application><application>Klotski</application></"
"application>, the following window is displayed."
https://gitlab.gnome.org/GNOME/gnome-klotski/-/blob/master/help/C/index.docbook#L8
https://gitlab.gnome.org/GNOME/gnome-klotski/-/blob/master/help/C/index.docbook#L234