Apple - How to disable accentuated char suggestion on long click on mac?

As given in the link by Tom Gewecke, you can disable the Character Picker by typing the following in a terminal :

defaults write -g ApplePressAndHoldEnabled -bool false

Open a terminal (e.g. Terminal.app) and write

defaults write -g ApplePressAndHoldEnabled -bool false

then press Enter.

Any open applications will still use the "Press and Hold", but if you quit them and then open them again they should work as expected (use key repeat instead of "Press and Hold").

Reference: http://m10lmac.blogspot.com/2011/07/os-x-107-lion-getting-rid-of-character.html