The dialog uses a wxStaticLabel, but this control does not recognize LF as a line breaker. It can be forced to wrap when a given width is exceded; to get this you must call the Wrap method of the control.
You can test modifying the code in confirmreplacedlg.cpp:38 from
XRCCTRL(*this, "lblMessage", wxStaticText)->SetLabel(label);
this->SetEscapeId(XRCID("btnCancel"));
to (untested)
XRCCTRL(*this, "lblMessage", wxStaticText)->SetLabel(label);
XRCCTRL(*this, "lblMessage", wxStaticText)->Wrap((GetWidth()*9)/10);
this->SetEscapeId(XRCID("btnCancel"));
Sorry. The width value should be get using
int Width, Height;
GetClientSize(&Width, &Height);
May be the sizer doesn't let the label grow vertically.