Confirmed.
$ LANG=it texmacs
TeXmacs] With linked TrueType support
terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid
[1]    6410 IOT instruction (core dumped)  LANG=it texmacs
But LANG=it_IT texmacs also doesn’t work for me.
$ LANG=it_IT texmacs                                                                               [11:42:41]
TeXmacs] With linked TrueType support
TeXmacs] system command failed: mktexpk --dpi 600 --bdpi 600 --mag 600/600 --destdir /home/huzf/.TeXmacs/fonts/pk ecss0
terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid
[1]    6866 IOT instruction (core dumped)  LANG=it_IT texmacs
Errors remain even when I locale-gen corresponding it_IT.UTF-8 UTF-8.
