Don't replace non default wxMessageDialog labels with default translations.

Don't replace non default wxMessageDialog labels with default translations.

The code translating the button labels of wxMessageBox to the application
language was overeager and replaced even the custom labels with the
translations of the default ones.

This shouldn't be done, of course, so simply check that no custom labels had
been set before translating.

See #10962.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69591 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775