Note: this may no longer be accurate as of Textadept 9.0. On keyboards with different locales, Ctrl+ß might be Ctrl+S (assuming the keyboard has an English 'S' instead of 'ß' on it). Similarly, Ctrl-[Cyrillic character] usually evaluates to Ctrl-[Latin equivalent].

To add locale dependent keyboard short cuts with value below 256, for example ß you could use something like:

-- Ctrl+ß
keys['ctrl+'..string.char(223)] = function()
  ui.statusbar_text = "Ctrl-ß works."
end