On some non-English systems running Linux, trying to type an accented character, for example with a tilde (~), will result in Emacs spitting out “<dead_tilde> is undefined”.

The root cause is that Emacs and some input method managers (ibus and SCIM) don’t work together; see Ubuntu bugs #1251176 and #493766. Here are some workarounds:

   (require 'iso-transl)
  env XMODIFIERS= emacs
  run_im none