The keyboard's plists of PressAndHold also can no longer be modified to add the new accents in El Capitan

From OS X 10.8 to 10.10, I used to modify the PressAndHold keyboard plists to add the new Romanian and Slavic accents and some symbols, so I'm an hyperglot.

/System/Library/Input Methods/PressAndHold.app/Contents

From 10.11, they can no longer be modified.

Files in /System are protected by System Integrity Protection. If you wish to edit such files, you will need to disable this.

To disable System Integrity Protection, you can do the following:

1. Boot into the Recovery HD by restarting whilst holding ⌘R.
2. Open Terminal (from the Utilities menu).

3. Run the following command in Terminal:

   csrutil disable
   

4. Restart.

^{Source: Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations}

You can re-enable it once you've edited the file.